Messengers need someone to host and maintain the servers. That requires a monetization model and requires someone to act as a central figure. That figure (like Facebook) then has to either moderate the content or try to make a case of not being able to but still facilitating it.

A messenger using the blockchain as its medium requires no staff to operate it and has nobody responsible for it. There is nobody to blame for whatever it's used for and nobody to dictate what it can and can't be used for. The group chats will simply exist and require no maintenance which is very desirable from an app point of view. Also a big positive is that you don't have to worry about the company running your messenger going out of business and then you lose the ability to use your app.