Because they abandoned Bagram in the worst way possible:

> US left Bagram without telling new commander: Afghan officials

> Before the Afghan army could take control of airfield, looters ransack barracks and rummage through storage tents.

That there sent a strong signal to both the Taliban and the ANA.

[1]: https://www.aljazeera.com/news/2021/7/5/us-left-bagram-airfi...

We already left Bagram. We did so overnight and it was a surprise to the Afghan forces we trained and left in charge there.

EDIT: And last month… people were pulling the wiring for copper among other savage https://www.msn.com/en-us/news/world/dollar800b-scrapyard-ba...

Bagram is about 25miles outside of Kabul, though.