Officially, it is because we ran out of purpose to go. Expensive as all hell to get there and further scientific value at the time was doubtful.
Unofficially… Have you ever played Battlezone?
Zbigniew?
Ever hear of a thing called Operation Cyclone?
https://amp.theguardian.com/world/1999/jan/17/yemen.islam
Zbigniew thought he would be cute and pose to the carter administration a fun little plot involving some old Nazi operatives, the Muslim Brotherhood (see Freya Stark and how that all ties into MI5), to fuck over the Soviets in Afghanistan. Of course, Congress had found out about the whole "the CIA has nazi militants on their payroll" scandal, so they had to rename them as the Mujahideen.
Our old boy Bin Laden came out of that whole fiasco.
A one Hillary Clinton was his protege, and she would later deploy the Muslim Brotherhood in Yugoslavia and then again in Yemen, Syria, and elsewhere.
This is why they were willing to commit treason to stop Trump. War crimes tribunals will see fully half of DC executed, I would imagine.