Is there any chance they don't yet have the tools to police it? It's taken a fair amount of time and resources for other platforms to do it, to the point that YouTube/Twitter rely on users to do so.
If you read interviews with Gab’s owner you will see there is no intention to mitigate the site’s extremism or radicalization of its members toward violence.
And when threatened, they've done so:
https://www.geekwire.com/2018/gab-user-deletes-anti-semitic-...
Again, seems to me we're holding to different standards.