In a round about way, "It's Bush's fault". I'm only being somewhat flippant.
Basically the Bush Administration, rather than being a career builder, ended up being a career ender for a lot of very talented "establishment" republican types, who left public service when, had the Bush Administration had gone better, would have been the next generation of leading Republicans. This opened the way for the Tea Party, the Fringe, the Ted Cruz types, starting in 2010.
Since around that time there was a sympathetic thread in the far right that saw Obama's liberalism in an extremely exaggerated sense, and saw in Putin and Russia, a "traditional values" counterweight to Obama. This, of course, speaks mostly to those individuals lack of education on Putin, on Russia and how international liberalism differs from domestic liberal politics. Putin's conservatism is not American conservatism, and is no ally of it.
Nevertheless, the Kremlin, always looking for useful idiots, has made overtures to that far right for years. I am of the right, and I've seen isolated fawning of Russia going back to 2011. Pre-Trump, the lunatic fringe only did it. It gradually became more mainstream.
Frankly, I think a lot on the right are desperate. They didn't know how to beat Obama. They don't know how to popularize themselves amidst declining demographics while simultaneously holding on to the aging voter blocs they dominate. They're trying to do anything except split the right, which frankly needs to happen.
There is a direct line, as I see it, between Republican's going from saying "we need to invite latinos in our party... they're religious and family values have a lot of overlap with us", to vilifying Mexican immigration, and trying to make nice with Russia. It's doing the easy thing - a further retreat from difficult choices - rather than doing the necessary thing, which is casting out some people who really need to be.