Because it's slightly/majority embarrassing. The groups backing this ban will find the worst of the worst thing, and paint you as a massive supporter. And their militant zealots will endanger your life if you become significant enough.

It takes a certain kind of person to stand up for pornography. And most are not that kind of person.

“The trouble with fighting for human freedom is that one spends most of one’s time defending scoundrels. For it is against scoundrels that oppressive laws are first aimed, and oppression must be stopped at the beginning if it is to be stopped at all.”

— Commonly attributed to H. L. Mencken (1880-1956)

You can defend porn while not defending incest.

Keep all the porn that is great. But there has to be a line somewhere and even the most accepting countries have a line.

Often the groups defending porn are defending the most egregious stuff which makes it hard for people to support.

The anti-capitalist left is disorganized and under-funded a priori.

Part of the left sees porn as abuse, so at a minimum they do not care about porn or may even support banning porn.