It had less than 1 million users in the middle of 2020. It's been out for only 2 years. There is no a long time ago for the service.
Those Trump supporters had to go to it to share their racist, violent, anti-Semitic horse shit because nobody wanted to do with the liability elsewhere. If you're going to defend white supremacists and racists then I'm going to assume you are sympathizer at the very least which means you're basically one of them.
Biden supporters aren't fervent supporters with flags waving from their trucks or property. They aren't starting violent coups. Nobody is a hardcore Biden supporter, lmfao. Terrible comparison, tbh.
You're forgetting one little thing with Parler too. These other services make money through ads. No company wants to touch Parler with a 10' pole. The company is currently losing money and has no way to make any revenue unless right wing donors give away their money to keep it afloat. They certainly aren't going to continue doing that.