Getting someone to onboard to a new platform is ultimately a sales job. Selling is about telling people who they want to be, not who they are.
A social media platform advertised as a politics-first/free-speech platform is the social media equivalent of a beer ad featuring a divorced balding man at last call in a dingy basement bar. No one aspires to bicker about politics with strangers on the internet, even though in reality that's the engagement that pays the bills.
It seems overly reductionist to ascribe the essence of 4chan to this one, singular idea. For example, m00t reports that 4chan was inspired by another image board he used to frequent. It is also hardly the first or only website to have light moderation.
I’m not sure whether it’s that, or that simply no one wants a platform everyone isn’t already on.
Personally I would absolutely not mind being “harassed” by text, if I was also able to exercise wide spectrum of free speech myself.