I've been wondering this for almost a week now.
Republicans lose nothing by moving on from Kavanaugh.
It's about Trump. Mitch McConnell didn't want Trump to pick Kavanaugh because he knew about the allegations/"baggage" towards Kavanaugh.
However, once Trump chose Kavanaugh, they had no choice but to go all in. Trump believes that Kavanaugh will save his ass from the Meuller investigation. Also, Trump was told by Roy Cohn back in the '70s or '80s, that if you give up in anyway, that means that you're "weak". Cohn told Trump to NEVER back down, even if it looks bad because at least you're not showing "weakness".