The success of a TV show can be gathered in real time. A movie needs to be complete before anyone knows about it.

So, they are going to make sure that movie is as sellable as it possibly can, because they only have one chance. The TV show can get tweaked as it goes, and sometimes milking a cash cow doesn’t result in quality. I think of Family Matters back in the 90s. It was a decent family sitcom, but EVERYBODY loved Urkel. So much so, that the last few seasons were essentially the Steve Urkel show. That’s who everybody wanted to see, but the show was complete trash. None of the storylines made sense anymore, and they had Urkel playing Urkel clones because MORE URKEL, MORE MONEY!

Or, of course, it loses popularity and then it gets canned.