Revealing mistake: When the guys are driving down the road, you can see Michael isn't under the truck. Yet 2 minutes later, he climbs in the bed. (01:16:25 - 01:18:10)
Halloween 4 (1988)
Plot summary
Directed by: Dwight H. Little
Starring: Donald Pleasence, Danielle Harris, Ellie Cornell, George P. Wilbur
Michael Myers is being transported to a new hospital, when it crashs he escapes, he then goes on another murderous rampage involving his niece.
More mistakes in Halloween 4
Earl: Martians could land on Ben's doorstep and all he'd do is spit once and get himself a shotgun.
Trivia: Due to an impending writer's strike, the script was completed in only 11 days by writer Alan B. McElroy. McElroy cracked the story, and managed to complete a final draft in this short period of time, reportedly turning it in only a few hours before the strike began.
Answer: Likely because keeping Michael in a maximum security facility was seen as a waste of time/resources given he had been in a coma for years.
TedStixon