Three things: the required bosses will be pretty difficult! You'll need to obtain items or more health from side bosses in order to beat them, extending the game.
-A game doesn't need to be particularly long to be fun. Undertale has only a few bosses but it still takes a while to beat and is fun.
-If I include every minorly significant boss as a required bossfight, it would suck the fun out of it because too many bosses mean you get bored of fighting them very quickly and the experience becomes stale.