"he repudiated support from David Duke a long time ago"

If "repudiation" can be summarized as "meh...", maybe he did.  But he had another chance to do so yesterday when asked that specific question.  Instead he just ignored the questioner.

Why?  Because, just as has been pointed out, those folks constitute a sizeable share of his base -- just like they flocked to the Teahadists when those know-nothings became a "hot item".  If it weren't for those jackboots, he'd never be able to stand.  He'd topple over.