The answer is PLOT.

They wanted to keep the original story going with a younger generation and the simplest way to do that is with younger family members. Also young people today don't want to see a senior citizen fighting off a knife wielding zombie man.

I agree with you though. To me Laurie would never allow herself to get pregnant or start a family because of the threat of Michael returning whenever the fuck he wants. The movie was OK and that's all it will ever be. There will never be another Halloween or slasher film that had the effects of the original. Same with Terminator. However, as longs as butts are in seats Hollywood will continue to bastardize and suffocate you with you things you enjoy until you gag at the thought of them.