Because we don't know the territory.
It's been time to go for awhile now. We're killing people and blowing up their homes, fields, and farms (which we think will pay off because it puts them in touch with the local government - yes, we're that far through the looking glass). They don't want us there, they don't know that this doesn't have anything to do with 9/11, and probably wouldn't care: they've got their own problems. We're doing counterinsurgency on behalf of a corrupt government that's probably playing both sides, and our main ally in the region, Pakistan, is unquestionably playing both sides. We don't speak the language, we don't know the players, our mission is unclear and our tactics seem divorced from what more or less appears to be our mission.
I was willing to hold fire for a little while on reports that our negotiations with the Taliban were getting somewhere. But we were just getting hustled, because we don't know WTF is going on.
Fuck this shit. We're not doing any good over there, we're not going to do any good over there, the people we're fighting against live there, know the territory, care more about it, and will be there after we're gone, no matter how long we stay. It's time to get the hell out of Dodge.
And the end of the fight is a tombstone white, with the name of the late deceased, And the epitaph drear: "A fool lies here who tried to hustle the East."
-Kipling
