For two reasons. (1) Michael's body was never found after the events of the first two films, which would contradict 4, 5, and 6 where he was in a coma then clearly reappeared. (2) It does not make much sense as to why Laurie would leave behind her daughter Jamie after faking her death but keep her son John who, in the timeline. would be only a year younger than Jamie. A scene in the original treatment established that 4-6 were in continuity by having a student give a report on the "Haddonfield Murders", talking about Michael hunting and eventually killing Jamie after which Keri goes to the bathroom to throw up, but this never made it into the final movie. The 2009 comic 'Halloween: 30 Years of Terror' contends that the story in Halloween 4, 5, and 6 are comic stories written by Tommy Doyle, the boy who Laurie babysat in the first film and who is a lead protagonist in the sixth film. He has a son named Jamie, whose name was given to the character Jamie Lloyd. In the movie series, however, the only real tie is that, in H4, Laurie is said to have died, but it's now explained that she faked her death and went into hiding.