Mainly because ISPs are not engaging in viewpoint discrimination. They don’t care what political viewpoint you have, as long as you pay them money, they will let you post it on the internet. In addition, most people have become used to the prices and expending a bunch of political energy to save $30/month isn’t worth it to a lot of people.

Now the feeling of being censored, on the other hand, generates a visceral reaction, and people can be convinced to expend a lot of political capital for that.