So if the party's attitude to the constitution has been deemed hostile and it does indeed have the potential to upend democracy in Germany this should be the perfect time to ban this party.
Problem being, when one voter every 5 supports a party, it's not that simple as you're basically saying 20% of your entire population is unfit according to the constitution.
It's a suicide, politically speaking
It may be a cynical argument but... People who wrote Grundgesetz doesn't need to be elected anymore, people who does politics today, instead, needs to.
We live in a democracy, at the very end, the electorate is the true final judge.