People who think they're right but have no traction look for traction. It's the Green problem; if you think ecosystem viability is the main issue but the practical effect of voting Green is splitting the left vote and electing people who will accelerate the trashing of the ecosystem you eventually get disillusioned with that and look for a route towards having a positive effect. The most promising route looks like Labour/the Dems, so you try that, and if you get anywhere at all then everyone else who is caught in the same bind notices.

I don't think you even need deliberate co-ordination for that to happen. I'm that kind of entryist in Labour at the moment; I joined when it looked like they were moving in a direction I agreed with, and I haven't got around to leaving yet despite them repudiating it, because there isn't a better option.

When I was at Oxford there was a legend about how the Conservative Association had organised to join the Liberal Society en masse, in order to have enough of a majority at the AGM to vote to close it down. Obviously that kind of entryism happens too, and in that case it's a conspiracy. But finding the most congenial home in a hostile environment isn't conspiracy. (Setting up your own home instead is fine in frontier territory, but stops working by about the third generation because the assets are all allocated. You can't, in the US, set up a viable third party without significant donor-class support, and the donor class are well served by the existing parties.)