I can’t comment on Nader specifically, but the 1980 “cutoff” date for decreased US regulatory activity is largely the result of Ronald Reagan winning the presidential election that November and the ascendancy (largely but not exclusively due to the latter) of the libertarian rooted economic conservative movement in Washington.

The story of the conservative movement is complex and features a great many characters, but it is important in understanding the rise of Reagan as first governor and later president. Essentially, the conservative movement slowly grew out of a fundamental opposition to the economic activist policies of the New Deal by a select few who began to advocate for restrained government action in general. Fueled by anti communist fervor and by what many perceived to be US foreign policy setbacks (later taking the shape of opposition to the Johnson Administration’s lackluster performance in Vietnam) and spearheaded by the efforts of dedicated intellectual conservatives such as Clarence Manion,the conservative cause transformed into a veritable movement in the late 50s and early 60s featuring a highly organized (but still largely decentralized - as seen with the rivalry between various groups who each considered themselves rightful conservatives (e.g., John Birch Society vs. YAF)) superstructure replete with think tanks, youth societies, political action committees, and media organs (e.g., the National Review). This movement’s first major initiative was the successful draft of Barry Goldwater for the Republican nomination for President, who subsequently lost in a landslide to LBJ in 1964 but did not take the conservative cause with him, a fact that would be made clear by Nixon’s win (with heavy appeal to conservatives and their new positions re: states rights and race) 4 years later and Republican control of the White House for the next quarter century with the brief exception of Carter who ill come to later.

Having campaigned hard for Goldwater in 64, Ronald Reagan, the charismatic ex-actor and New Deal democrat turned conservative Republican, became a popular face among conservatives. This association with the movement enabled Reagan to win the governorship of California in 1967 and hold the office until 1975 (at the time California and much of the West was a Republican bastion and a fairly conservative state outside of some liberal circles such as Berkeley and the Bay). While not putting into effect much of the conservative economic agenda in California (he actually raised some taxes and increased spending) conservatives saw success on the social front as Reagan distanced himself from homosexuals, refused to enact liberal abortion laws, and cracked down on anti war protests which were becoming increasingly concerning to conservatives.

In between Reagan’s governorship and his final and successful bid for the presidency in 1980, a couple of important developments occurred that greatly diminished the economic and foreign policy situation facing the US, all under the first democratic president in nearly a decade, Jimmy Carter. First and perhaps most important was the bastard economy that existed throughout the 1970s called “stagflation” (a stagnant or a depressed economy with heavy and sometimes rampant inflation - an apparent violation of economic norms which traditionally see inflation occurring in times of growth and deflation in times of recession). While explanations for this stagflation are many and diverse (the inflation caused by the end of the gold standard by Nixon and the 1973 Arab Oil Embargo mixed with government price controls reducing growth and leading to layoffs as companies dealt with higher input costs without being able to offset these in higher prices, being a popular explanation) what is sure is that it represented an economy in recession which was unable to be repaired by the New Deal-esque government intervention called for by Keynesian philosophy and espoused by most old school democrats. As such, the public’s faith in the regulatory and activist state decreased in the 1970s laying the groundwork for Reagan’s economic conservative run in 1980. Adding to this situation was an apparent softness of the Carter administration abroad that enabled communist gains in the 3rd world, most notably the USSR’s (up until then largely successful) invasion of Afghanistan in 1979, and an erosion of American influence more generally, most notably the 1979 Iranian revolution which deposed a major ally and the subsequent hostage crisis which continued to dog Carter’s late presidency and was a major factor in American antipathy towards his administration.

Together, all of these enabled Reagan to win a relatively easy (few Republican challengers were able to more than superficially contend with him in the Republican primaries, with the most significant being his later pick for VP, HW Bush) and famous land slide victory in 1980 which importantly led to a substantial coattail effect and a heavy Republican win in the Congressional elections, giving the party, largely bolstered by the conservative agenda, their first majority in the Senate since the Eisenhower’s first term and their longest hold (6 years) since before Great Depression.

Once Reagan entered office, he essentially changed the game in terms of regulatory policy and taxation. He substantially reduced the amount of federal regulations during his time, illustrated by the fact that the number of pages in the Federal Register - a rough barometer of the extent of federal regulations - was reduced from 87,012 in 1980 to 53,376 by the essential end of his second term in 1988. The improving economy under Reagan after the 1982 recession in turn led to a new public perception of government regulations as hampering growth, itself translating into a reticence for advocating for such policies in congress for fear of voter backlash. Similar situations occurred with taxes which became a taboo issue and are credited with costing HW Bush a second term after he broke his campaign promise and raised taxes in an 1990 omnibus appropriations bill to reduce deficits. Indeed, Reagan’s legacy has loomed large with the new breed of democrats (even down to Joe Biden today who only claimed he would raise taxes for the very rich making over $400k) to eschew large scale regulations and tax increases, as seen with Clinton’s famous declaration that “the era of big government is over” and even Obama having to mask his tax increase in the language of the ACA’s “individual mandate.”

Sorry for any formatting errors, on my phone. Sources and Further Reading:

Andrew Busch. Reagan’s Victory (for the 1980 election and the immediate background and a brief overview of Reagan’s Presidency)

Rick Perlstein. Before the Storm (for Goldwater and the conservative movement).

Edit: corrected Reagan’s coattail effect, Republicans only gained control of the Senate, not the House, although they gained significantly in the latter as well.

Edit: thanks for the silver kind people of Reddit!