Good point. 'Rats also have a tougher time marginalizing their extremists, even when/if they would wish to, because liberals in the 60's and 70's foolishly went along in adopting the (communists devised) taboo against "red baiting". Thus it became presumptively improper to even identify left extremists as such, let alone exclude or marginalize them.
At the same time marking and marginalizing right extremists is socially acceptible and socially reinforced. Thus we have the effect that your typical library wouldn't think of stocking a book written by David Duke, but probably has dozens by Noam Chompsky who is easily as far to extreme left as Duke is to the right (and not much less antisemitic).
Much as we might justly complain about the double standard involved, it's actually benefited conservatism and Republicans in the long. We have come to be seen as having our extremists in check, whereas the left loons are running the asylum. Thus moderates and independents are more likely to trust, and vote for, conservatives.
So it will remain, unless creationists become too visible. But I fear that cat's already out of the bag. Now the whole game is to keep the lid on.