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.
Other mistake: When Doctor Loomis encounters Michael in the gas station the camera zooms in on Michael standing directly in the kitchen and Loomis shoots at him, but when he takes a closer look he is totally surprised Michael vanished when he saw him.
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