I would guess the Gerry/Sylvia divorce or Lew Grade pulled the plug on it?
Unfortunately, this major motion picture was budgeted at $36 million...twice the budget for an entire season of Space:1999. He wanted it badly though, so it was the end of Alpha and the gang.
Lew Grade's pride and joy opened in the summer of 1980 and made a whopping....$7 million. The title of this debacle?
Raise The Titanic.