Other mistake: In the scene where Jamie is in the closet, looking at a photo of her mother (Jamie Lee Curtis). The photo looks exactly like the scene where Laurie is sitting waiting for Annie on the ledge in the original Halloween, which wasn't photographed at the time, it's clearly a still from the movie.
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
Dr. Loomis: I need to speak with Sheriff Brackett.
Deputy Pierce: Well, then you'll have to travel about three-thousand miles south of here.
Dr. Loomis: What?
Deputy Pierce: Brackett retired in '81. He went to St. Petersburg.
Trivia: Series creator John Carpenter actually co-wrote an early draft of the script and even considered directing, but his ideas were dismissed by producers as being "too cerebral" for audiences, and he subsequently dropped out of the project. His script reportedly was more focused on the psychological effect Michael Myers had on the residents of Haddonfield (and possibly inspiring other murders), with Michael only appearing in visions, dreams and possibly as a "spirit."
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