This could either be a conspiracy comprising of every author of every medium...

OR

It's because stories need sympathetic characters, and current Republican values aren't sympathetic.

I'm not trying to be divisive or flippant, though I'm sure this will be the effect. But success in media is driven by believable sympathetic characters (alternatively explosions), the only two that currently come to mind was John Goodmans character in Alpha House, and Ron Swanson (clearly libertarian) in Parks & Recreation, both of whom are very sympathetic characters, that put people before politics, and neither of whom would accept the last couple of years of their party's behavior.

TL;DR: It's either caused by a vast conspiracy, or the effect of what works in story telling, whichever you think is more reasonable.