There are a host of reasons, and it'd be nearly impossible to give an accurate account of each and every factor as well as the degree to which they individually influenced the stagnation of income. I actually took an entire year long seminar just on this question alone, and it's an area in the study of political economy that is still very active in determining the entire field of reasons. So what follows is my attempt to essay the broad reasons typically given. I'll include a number of texts which you might wish to reference at the bottom of my response, but due to the sheer breadth of your question, I won't cite specific pages because I'm often summarizing entire books and even volumes of work.
But the general list most economists and sociologists have agreed upon includes (in no particular order):
The increased international competition in large-scale sectors such as steel and auto put pressure on national companies, leading them to layoff workers, relocate factories, outsource, etc. (recall that much of Western Europe had sufficiently rebuilt from WWII by the 1970s).
Similarly, unions were increasingly being seen as burdensome and too rigid to respond to the new demands of business; and as firms became less inclined to bargain with unions, union members were increasingly losing the benefits associated with unionization—notably in this case, rising wages. Unions themselves were frequently embroiled in massive corruption, which meant they were inefficient and in some instances frankly parasitic—doing things like embezzling from their members' pension funds, political racketeering and extortion, etc. So unions were losing a lot of their legitimacy in the eyes of the public, which is not good for workers.
Also recall that the US economy in general was experiencing both stagnation and inflation, which were exacerbated by the OPEC crisis. Economic crises do not bode well for the average worker in terms of increased wages. The effects of stagflation also helped usher in the political-economic phenomenon of neoliberalism, which is a broad, very loosely defined concept. But basically it ran counter to the distinguishing elements of the post-War US economy, in that, e.g., tax breaks were given to large corporations, unions were seen as almost wholly negative for the economy, there became increasing emphasis on quarter-to-quarter growth, with less regard to five- and ten-year growth (which has all sorts of implications for things like workers morale, pay, etc. because of the pressures imposed upon the firm's executives by public shareholders).
We can also look to more political changes, such as the massive increase in targeted and specific national and state legislative advocacy for each sector of the economy. And union advocacy on behalf of workers is at this time going down, so the advocates for, say, the growing finance economy aren't speaking on behalf of Joe Briefcase.
To expand upon neoliberalism, particularly as it relates to the change in finance, here's an edit:
I suspect the reason OP chose the 1970s as the starting point (and why I in fact confined my response, loosely, to that decade) is because of the second graph on this news report:
http://voices.washingtonpost.com/ezra-klein/2011/01/a_graph_im_trying_to_understan.html
(There are loads of other similar graphs out there I just chose the first one I saw on a Google search.)
I absolutely agree that the changing forces at play during the '70s were made more potent by the trends in the '80s, especially following Reagan. Were I to expand on any of the points made in my previous post in this respect, I'd lump the additional phenomenon you mention under the umbrella term of the rise of neoliberalism.
I guess I ought to expand upon neoliberalism. On a purely domestic level, the change in corporate structures which began in the seventies and were heightened in the eighties increasingly made executives and higher management more accountable to shareholders. This had the effect of making quarterly earnings the largest priority for any firm (so as to maximize short-term growth, the theoretical idea being that this is also good for the long-term growth of the firm).
Were I to put a finger on the biggest factor in the growth and hegemony of neoliberalism today, I would point to the structural changes that occurred in the relationship between the state and the finance sector. Prior to 1975, the finance sector was relegated to being a body whose sole purpose was to provide capital and liquidity to firms operating under the Keynesian model of private and state relations. Though I'm simplifying, the finance sector was seen largely as a mere means to an end (the end being the long-term growth of largely scaled industrial firms—e.g., steel, auto, and oil). Prior to '75, the Security and Exchange Commission (SEC), which was the regulatory body established following the Great Depression to regulate the trade of financial security products (for instance, insurance) required finance brokers to adhere to a pre-established commission fee. Post-1975, the SEC agreed with institutional investors that the brokerage fees were hampering larger economic growth (recall, economic times were hard). This more or less enabled the rise of so-called 'shadow banking' firms to emerge, which weren't hampered by the depository interest caps established in the 1935 banking reform act. Both financial and non-financial firms were able to move their savings out of traditional commercial savings accounts into money market funds and mutual funds, which offered higher interest yield rates.
And so by 1980, Congress passed the Depository Institutions Deregulation and Monetary Control Act, which removed depository interest rates from commercial banks and allowed commercial banks to engage in speculative finance (so not just savings and lending). Which meant that the wall that had been erected post-Depression was completely removed.
So now, firms that were once discretely involved in traditional banking (savings and low-risk lending) were one and the same with investment firms that deal with way higher speculative risk.
The total effect of this is that it redoubled down on the non-finance sectors to operate more in line with the short term, higher risk models of business conduct.
Take this in combo with the fact of Reagan slashing taxes, and it's apparent that the state's interest was more involved in promoting the growth of large corporate business (operating in line with the new deregulated model) than with promoting better working conditions for Joe Briefcase.
By the time of Clinton, the paradigm shift that had confined itself only to the economy was manifest in social programs enacted by the state. See the 'neoliberal' changes in welfare and food stamp programs as an example.
Did I connect the dots well enough, here?
So there's a decent summary, though not exhaustive. I'd look to the following books for more detailed investigation than what I am putting forth here:
Timothy Noah, The Great Divergence, Bennet Harrison and Barry Bluestone, The Great U-Turn, Corporate Restructuring and the Polarizing of America, and William Tabb, The Restructuring of Capitalism in Our Time.
If you're interested in big-picture, sort of comparative models of political economy (which I sure am), then take a gander at Contemporary Capitalism and Its Crises, eds. Terrance McDonough et al. This collection of essays come out of a school of economy that works with a model called Social Structure of Accumulation Theory, which, boring as it may sound, offers a historical analysis of the various affective socio-economic institutions in a given society and how each institution's erection, evolution, and decay—when taken in a dialectical whole with all the others—produces a distinct socio-economic epoch. The decade of the seventies represents the 'crisis' stage of the Third Social Structure of Accumulation in American History, which began c.1940 and lasted until c. 1980. With the next stage being our (arguably) current one.
That's quite an interesting question, and a very complex one as well. I've heard a variety of explanations from a variety of different sources. The Economic Policy Institute claims that "abandonment of full employment as a main objective of economic policymaking, declining union density, various labor market policies and business practices, policies that have allowed CEOs and finance executives to capture ever larger shares of economic growth, and globalization policies. Collectively, these policy decisions have shifted economic power away from low- and middle-wage workers and toward corporate owners and managers." (http://www.epi.org/publication/causes-of-wage-stagnation/)
Unions and their supporters tend to concur with this assessment, as this post from the AFL-CIO makes many of the same points: http://www.aflcio.org/Blog/Economy/Five-Causes-of-Wage-Stagnation-in-the-United-States
On the other hand, Michael Greenstone of MIT and his colleague at the Brookings Institute, Adam Looney, might emphasize the growing importance of education in the workforce since the 1970s as one reason incomes have been falling, as lower-and-middle class people, particularly men, without college educations have found their wages declining steeply since that decade:
http://economix.blogs.nytimes.com/2012/10/22/the-uncomfortable-truth-about-american-wages/?_r=0
"Among the most robust findings in economics is that education reduces unemployment and increases earnings. But even with the remarkable capacity for education to produce growth, the rate of educational attainment in the United States has slowed, especially for men. The share of men 25 to 34 with a college degree, for example, has barely increased over the last 30 years. (The trends are much better for women.) The United States, once the world leader in educational attainment, has been surpassed by many countries."
Pew Research, on the other hand, offers a few other explanations:
"One theory is that rising benefit costs — particularly employer-provided health insurance — may be constraining employers’ ability or willingness to raise wages. According to BLS-generated cost indexes for wages/salaries and total benefits, benefit costs have risen about 60% since 2001 (when the data series began), versus about 37% for wage and salary costs. (Those indexes do not take inflation into account.)
Other factors that have been suggested include continued labor-market slack; lagging educational attainment relative to other countries; and a broad decline in better-paying jobs and consequent shift toward job growth in low-wage industries."
Finally, though I haven't read the book yet, one maybe interesting source I've found is "Divergent Paths: Economic Mobility in the New American Labor Market: Economic Mobility in the New American Labor Market" by Annette Bernhardt, Martina Morris, Mark S. Handcock, Marc A. Scott, published by the Russell Sage Foundation in 2001. From what I've gathered several sections of it deal with wage stagnation, you might find it useful.
Again, this subject is a bit too complex and broad for me to feel very comforting putting forward any firm hypotheses or explanations of my own, but I hope the sources here can give you some ideas on how to start off and perhaps guide you towards finding more experts to help answer your questions.
Follow-up question: many measures of US well-being have increased substantially since the 1970s:
more varied retail options, such as larger supermarkets with a larger variety of goods
widespread availability of inexpensive air transportation
cars that are safer than ever, with more comforts such as superior in-car entertainment, power accessories, and air conditioning
more life-saving technologies available in health care
increased average home sizes, with newer homes having better features such as air conditioning and even fire sprinklers
explosion in pervasive wireless communications, with cheaply available pocketable supercomputers and the limitless nationwide voice communication
widespread availability of inexpensive multi-channel home entertainment, with explosive growth in the variety of serial television programs, news programs, and sports coverage
We typically use income as a proxy for average well-being. In the face of such improvements in well-being since the 1970s, have incomes become a flawed measure for well-being? Or, have all these technological improvements somehow been overshadowed by something that has actually made the middle class worse off despite these improvements?