No, Pooter, they didn't get new writers, but you would think so just because of the way the stories differ. I have to agree with you that the characters of the past have much more going on than the characters of the present. Maybe that's not quite all true though. Maybe it's just that the present day trials and tribulations of our favorite family for significantly more boring than those of the past.
You'll see in an upcoming time-travel sequence that the story is very broad and all-encompassing, focusing around one main character for the most part, and when originally aired, it lasted (I believe) 9 months.
It does seem that more time went into the planning of the time travel areas of the show, at least, the first two for sure, because theyir stories were elaborate and involved. Unfortunately, you will also come to realize that after the initial wonderfulness of these period wears off, that the writers attempted to stretch out the stories in the past because they were immensely popular, and then fall very flat compared to the earlier stories.