Barbarian: our first Barbarian was a Totem barb. I haven't seen any other subclass that motivates us to return to that class.
Bard: very squishy. The Inspiration die is almost game-breakingly good, but it's no fun to bring along a "super die" the others have to constantly protect.
Cleric: we've used Light and Tempest to good effect. Problem is, now we're running out of first-tier results. Also, very few cleric-specific feats and no good multiclassing makes Cleric a done deal, a problem solved.
Fighter: There are really only two options: Battlemaster with Precision maneuvers, and Eldritch Knight with Shields. I'm sure a case can be made for a few more builds, but then again I consider Fighter one of the most successful classes of all of 5E. The multiclassing potential is wonderful; many feats are tailor-made for fighters.
Valor Bards and Blade Warlocks: the problem is that compared to real martials, you're simply way too squishy. Besides, you will never use your weapons, since your spells will outcompete weapon usage every time.
...and so on, you get my point.
Adding more subclasses might be good for players looking for a specific archetype, but almost none of them are interesting in the least from a charbuild perspective.