The problem is that the alt-right has just proven useful for the Republican party.
Donald Trump is much more likely to sign bills coming out of the Republican Congress than Hillary Clinton would have been. The whole wishlist is now in play: upper income tax cuts, capital gains tax cuts, repealing Dodd-Frank, repealing Obamacare, you name it.
I’m not saying these will definitely happen, just that a Trump presidency and Republican Congressional majorities make them possible. For that, Republicans have the alt-right (among others) to thank.
Add this political incentive to the belief, held by some on the right, that “racist” is just what the left calls people they don’t agree with, and the chances of left, right and center joining together to denounce the alt-right are extremely small.