Earlier quoted context omitted.
Because the people there are far right. When you go to a website and the first thing you see is calls to murder jewish people and the n-word repeated ad nauseam, all but the far right decide to just leave. The end result is that gab/parler/etc don't become places where all speech exists but instead places where extreme racism, homophobia, and sexism suck the oxygen away from everything else.
This is a false depiction. I haven’t used Parler much but checked it out a few times and did not encounter a single piece of content advocating violence, talking about Jewish people, using the n word, or partaking in any kind of racism. Your claim that these are repeated ad nauseum suggests you never actually visited any of these sites and are just criticizing them without basis.
The court documents (https://www.courtlistener.com/docket/29095511/parler-llc-v-a...) from their case against Amazon contain other examples of hate speech.