It's literally not censorship to voluntarily refuse to associate with white supremacists (or to simply have no interest in doing so). I'd assume this is the reason we Americans are not using Gab.
That would hold more water if people were using services that are not heavily censored, or not censored at all. The white supremacists (apparently, I've not used Gab) all used to be on Twitter - were Twitter users associating with them back then? Did everyone stop using Twitter?