Notwithstanding the moderator's reply removing the question, the broader question can be answered in a historical way, so I will rephrase the question to be historically appropriate and then answer it: Why does there appear to have been a large number of congressional party-switchers in the 1980s and 1990s? If the moderators would be willing to restore the question and allow it to be reframed, I think that would be useful.

I won't run back 160 years of political parties, but let's start around 1950. In 1950, the Democrats are the party of the New Deal. In the 1920s Republicans shied away from government intervention, the Great Depression occurs, and after a fashion Roosevelt is able to deliver the New Deal. At this point, Democrats are generally progressive. The term progressive is used in several ways, but in this paragraph I mean that they believe there is a large role for government to provide services and to intervene in the economy to help protect consumers and workers and invest in targeted areas. They also have huge majorities in congress. They are associated with labor. Republicans are largely conservative, and mostly located in the north of the country. They are associated with business. When Republicans win the presidency -- just Eisenhower in this period -- it's because they are not strongly associated with debate around the role of government. Democrats have a real stranglehold.

Under Kennedy and later Johnson, Democrats began enacting reforms related to civil rights. Politics changed materially as a result of this. Republicans in the North generally supported civil rights, as did Northern Democrats. But Democrats in the south opposed civil rights. In Congress, you had quite a few votes that were bipartisan but polarized along geographical region. When a new "cleavage" (thing people can disagree about) emerges in a political system, there is major electoral pressure to re-align parties around capturing this cleavage. Civil rights suddenly became the hot topic, but it had an unusual relationship with your partisan label.

Beginning with Richard Nixon -- you can look up the Southern strategy -- Republicans progressively made inroads in the south, adopted conservative positions on civil rights, and basically formed the modern party coalitions we see today. You can see that mapping up the positions on civil rights to the positions on the economy involved changing what each party stood for and which kind of voters they pursued.

In the 1970s and 1980s this was already visible at the coalitions that supported people at the presidential level, but there was still a lot of inertia in state level parties and in the house and senate. For a long time, increasingly conservative Democrats who were increasingly out of touch with the national Democratic party continued to dominate the south's state, local, and congressional politics.

Let's think of a modern example. Imagine you're a Republican and you want to go into politics, but you live in Hawai'i. You decide to run for State Senate. Democrats have 24/25 state senate seats today. Are you going to get elected? No, of course not. You're toast. Instead, the smart way to get elected is to be a Democrat, even if you're a conservative Democrat. So you join the Democratic party and try to move it right.

So it was in the 60s, 70s, and into the 80s and 90s in the south -- the Democratic party had dominant positions in every southern state. Many elections in the south only had a single candidate.

The process of figuring out how particular ideas map to particular parties is called partisan sorting. When you take a political compass quiz, you might learn that your values make you closer to the Democratic or Republican party. But for many people in the 1950s and 1960s, they didn't have this information. They often inherited partisan memberships from their parents, their local communities, etc. Party label was not specifically all that informative and voters had a lot less information to understand how parties mapped to ideas. Also as I said, when civil rights became a big part of politics, it wasn't even obvious that how your local member thought about these issues had any connection to their party. Politics was also a lot less nationalized, so it was very easy for people to vote for one party locally and another nationally, something that's a lot less common today. The 1980s is about the period where the dam breaks.

The dam breaks for two reasons:

(1) Republicans begin winning more, and building up their brand and their party in southern states. It becomes less of a bad thing to be a Republican, and conservative Democrats who might otherwise be Republicans switch.

(2) After 1968, Democratic presidential candidates tend to run on more muscular left-wing platforms (there's a lot behind this, including the mechanism of how the Democrats select presidents, protests at the '68 DNC, and other things).

Observe that almost all of the 1980s examples in the list you link are Democrats in the South becoming Republican. This process continues past the horizon of the 20-year rule for this sub, and actually it accelerates in some regards under Obama. Kim Davis, the Kentucky clerk who refused to sign off on same-sex marriage and got in the national news... was a registered Democrat. Even though nothing about her matches the party she was elected under. This kind of process takes time.

The reason there's less party switching in the period past the horizon of the 20-year rule for this sub is because partisan sorting is nearly complete. People know if they are Democrats or Republicans. Geographies are highly sorted -- so you also know if you live in a Democratic or Republican state. Who you are, in terms of education, class, race, religion, and location where you live gives you a clear path to pick a party. So it is too for representatives who are elected for those parties. This will not be true forever, but it does seem to be true now.

You still see party switching sometimes. I will refrain from commenting too much on the present, but you would expect party switching to occur when someone is badly out of touch with their party as a whole, and where they expect the other party to win and thus would gain advantage from switching. A recent example is West Virginia governor Jim Justice.

Sourcing:

All of these ideas intersect with political science and political history, though I've said very little that requires specific sourcing. For a view of what voting was like in the 1950s and 1960s, try The American Voter by Campbell, Converse, Miller, and Stokes, which discusses how political scientists thought about voters making decisions. For work on political polarization -- the phenomenon that parties drift apart over this time period -- try McCarty, Poole and Rosenthal's Polarized America. Poole and Rosenthal are also the developers of the main method we use to score political ideology in congressional voting, NOMINATE. If you want a general overview of how the relationship between parties and ideas has changed, you might look for work in an undergraduate text on "party systems" in the U.S. -- political scientists differ but would generally label the period we're discussing the fifth or sixth party system of the U.S. Ken Martis, the political geographer at WVU, has a really beautiful bound volume called something like The Historical Atlas of the United States Congress which shows colour maps based on partisan control in Congress over the entirety of American history, and just flipping page to page really helps you see how the relationship between party and geography changes, creating the kind of incentives I discuss above. I avoid linking my own work.

This is a pretty foundational question and so really any undergraduate review of political history of the 20c will substantiate everything above.