Not just animation fans. This applies to live-action, too. Did anybody even care when Modern Family ended? That show used to be BIG, but it went out in a silent puff of smoke.
Some major reasons why streaming prevails are the lighter censorship, the bigger budgets, and the encouragement of serialization. You can't get that with sitcoms. The most recent sitcom I can remember being even moderately popular was The Goode Place, and you can guess why that caught on better than others.