Remember that they impeached Clinton, over sex, after an investigation into his financial affairs went nowhere. The goal was impeachment; the reason would come later.

For Obama, they had his race, and that was sufficient. Not every Republican is a frothing racist, but they're certainly happy to appeal to the frothing racists. But at core, it was simply that he was a Democrat. Democrats are inherently polarizing because they are not Republicans, and the reasons come later.