Because they were seen as lesser artistic outlets than cinema due to the rigidity of television schedules and advertising. These days, it's the Netflix series that gives you the most artistic freedom. You can make any number of episodes of any length to tell your story and you're free to appeal to a niche audience. Movies are forced into a relative narrow length window and business constraints force you to appeal to an international audience.

The creativity is still there. It's just in a different place.