Didn’t help that he showed up at his first couple Senate meetings gloating about winning the election. And tried to set working hours for his former colleagues. Among other things.
Obstructionism as a core tenet of the (former) Republican platform is reprehensible, and retrospectively probably led to quite a bit of discontent with the government’s inability to address problems that Americans face. That same discontent fomented the current reactionary swing, so in the end maybe they really got what they wanted. Shameful.
"Other things" most obviously being the racism caused in part by significant cognitive dissonance that uniquely affects white supremacists when having a black president.
They hated him for what he was.