In addition to what others have commented, communist and all other flavors of political parties are protected under the german constitution as long as they aren’t anti-democratic or call for violations of basic human rights. That’s because the right to form a party and express your political opinion is also protected in the constitution. So ironically it is really hard to ban fascist parties because the highest court would have to prove that their exercising their freedom to form a political party is in conflict with other basic rights and freedoms.