And how exactly would you define it to ban it? Germany already has some of the most extensive hate speech / anti-extremism laws in Europe. I mean, it's not just censorship, but e.g. outright bans on parties expressing certain views in their platform, or even just insufficiently democratic internally, and with various specialized institutions that enforce all that proactively. And all that wasn't enough to prevent AfD from operating legally.
Facebook can just ban them by name without any explanation, of course. Are you suggesting that it's a good thing - that, because we seemingly can't write fair laws on this, we should delegate both the laws and their enforcement to private industry, where it can be done without any checks and balances?