This has nothing to do with Kavanaugh being moderate or extreme. It has to do with the politicization of the judicial nomination process that has been going on for decades. Kagan and Sotomayor didn't get Republican votes because they're mainstream; they got Republican votes because they were replacing similarly liberal justices and many Republicans subscribed to the notion that elections have consequences and that a qualified, non-crazy nominee should get confirmed.
Kavanaugh, on the other hand, is replacing the "swing" justice, at least on some issues. (I'm constantly amazed how many people are worried that Kavanaugh is going to kill Obamacare without realizing that Kennedy voted to do just that and Kavanaugh has twice voted NOT to.) Crucially, he's replacing the only "conservative" who would likely vote to uphold Roe, which is behind a lot of the hysteria. He's Trump's nominee in an era of "the resistance." And Democrats are still furious about Garland (and they have a point, though Republicans also have a right to be mad about Miguel Estrada and others). This is why no Democrats will vote for him.
He has a political background, but he has since distinguished himself as one of the smartest judges in the country. He may not be quite on the genius level of a Kozinski or Posner, but that's probably a good thing as geniuses have a high propensity to be a bit nuts. In fact, the only judge in the country I can think of who's both a "legal genius" and relatively normal is Judge Boudin, but has health issues and no longer hears cases.