Conservatives in 2008 were almost exclusively white

Exactly that's my point, and the supermajority of non-whites voted for Obama, while the majority of white voters who voted went for McCain, if ideological commitment formed the core of election outcomes then that wouldn't have been the case, and we would see a more even distribution among demographic groups

The majority of McCain and white Obama voters voted the way they did because Obama was black, the conservatives voted the way they did because he was liberal and to be liberal is to be black or support black causes, and white Obama voters (aside from the ones who genuinely didn't care he was black) voted for him because voting for the first black president would've been a cultural signifier they could use as social capital in the more cosmopolitan spaces they inhabit

White Obama voters who voted a second time couldn't have cared less about his ideological promises because he betrayed all of them in his first term