we attacked them....
they never started the war...
the taliban did not finance, train and supply the people who were involved in 911 (as far as I know but I may be wrong)
You don't actually know any of that as fact. The public simply doesn't have that kind of information.
Besides 9/11 was not the only terrorist action that has happened.
What we do know, is that the Taliban Regime have and continue to help finance and support various Terrorist and Islamist groups including al-Qaeda and others.
If they are allowed to regain power in the region, it could and in all likelihood would further destabilize the entire region. When you have nuclear powers that are at risk of insurgency and a failure of statehood then allowing a State that will actively support and encourage that failure is something we should all be extremely concerned with.