The Afghanistan Taliban training camps are still operational, don't need another organized 9-11 attack.
Afghanistan is in an undeclared war with Pakistan, who has nuclear weapons. (strategic & tactical).
Pakistan has issues with India who has nuclear weapons. (strategic & tactical).
India has an ongoing border dispute with China, who has nuclear weapons. (strategic & tactical).
China is building islands in the China sea expanding their sovereign borders & funding North Korea.
North Korea has nuclear weapons, and says they will use them against the United States someday.
Afghanistan also borders Iran which says it can have weapons grade uranium in 5 more days.
Iran has also declared they want to remove the state of Israel from the face of the earth.
Iran has also moved forces into Iraq, Syria, and Jordan, with the assistance of the Russians.
Besides that, can't think of many more reasons why we might wish to stay in the area.
Unless the Russians wish to invade Afghanistan again after we pull out.