I was gonna respond with "cause conservatives know they're bigots xo" but if you want a serious response:
I think a lot of it has to do with the polarization of politics in the US that has been happening since the turn of the century.
It's interesting because both the left and the right were very close to some extent, ie: why people even in the 90s talked about the working across the aisle, etc. The right started to go more to the right and the left only started moving farther left 2008, but now they're really away from each other. See:
I have no clue why, really. I have some theories. I think the failure of Republican fiscal policies has put them in a tough place. GWB basically had to resort to blind patriotism to get the public on his side since Clinton had an arguably favorable presidency amongst most people. And even then, it backfired on him.
I think the failure of trickle down economics (and the American people, at least temporarily, realizing it fails) eventually lead Republicans to have to embrace a side of them that is radicalized. The Tea Party is wild as hell and a weird subset of Republicans who, sadly enough, were powerful since Republicans always vote in midterms more than Democrats. They pushed the party more right, and because they're extremely conservative socially, more and more the Republican party started alienating even bases that would go for them like fiscally conservative Asians and Latinos or even college-educated whites.
Essentially a line in the sand was drawn and now both parties work to be the opposite of one another, which in turn is going to be extremely hostile and polarization will cause resentment because most topics now regarding politics are loaded and emotion-fueled for many people.
Basically - US politics became less about the way to lead the country and more about a debate on people's entire identities and self (obviously that has been going on for decades for certain minorities, but now the attitude is overly present)