If you have one platform which bans, say, racists, and another platform which doesn't, then most normal people will gravitate to the sans-racist platform. The sales team of the latter platform then has to go out and convince people that they want to advertise to a small market of racists and racism appreciators. This, fairly obviously, isn't as easy as selling ad-space on the less racism-y platform.

People have _tried_ unmoderated forums and other social media, again and again. It doesn't work. Being unappealing to advertisers is actually the _good_ case; the bad case is that people use it to orchestrate serious crimes.