Because sometimes, the parties to be banned, get too big, too fast to just squash.
AfD has enough popular support than banning them would probably turn Germany into France or worse. The riots would be pretty intense, and it would probably push fence sitters off the fence to the right.
So the establishment in Germany is desperately trying to convince everyone AfD is a hate group, and when that idea has enough popular support, they can enact the ban.
This is what happens with larger groups the establishment wants gone. They have to discredit and weaken before they can actually smash.
You can't just ban parties in most Western Countries. You have to have a 'legit' reason.
And sometimes just the right media can do it and a ban doesn't actually have to be enacted. At that point the ban is an effective social taboo.
This is how the game is played, Zach. And it has been played like this since before the Enlightenment.