Because I don't need a paladin (for example). When people say they'd want the paladin removed because "you can just play a fighter/cleric multiclass"... I don't necessarily disagree. But the reason to have a paladin class is because it gives new and different mechanics than what you get as a fighter/cleric. Which is fine. Having some additional mechanics beyond the Basic Rules doesn't bother me, so the fact they went up to twelve classes in the PH doesn't bother me either. And if/when they decide to add one or two more-- the mystic, the artificer-- that's cool.
So, "this far, but no further?"
I'm dubious.
At least those two have a legitimate story to them and their stories are grand enough than they CAN support multiple sub-classes (so in that regard, I think them being full classes are fine.)
Actually I doubt the artificer could fill out a whole lotta sub-classes, even the gunman one seems a tad forced. They Mystic, OTOH, is going to be combining a half-dozen or so psionic classes from several past editions, so should have plenty to work with; the Warlord had 8 de-facto builds & 28 Paragon Paths; the Shaman 5 & 17 ... the Artificer was in Eberron in 3e & 4e, the 4e version with only 3 builds & 3 Paragon Paths, for comparison. Not that the Artificer shouldn't be a class, just that filling out a lot of sub-classes may not be its strongest suit.
And, has been pointed out, some extant 5e PH classes have a paucity of plausible/distinctive (let alone 'grand') sub-class 'stories.'
But I just don't want to see a proliferation a la 3E (especially when you bring prestige classes into the picture) because they don't tend to be used because their stories warrant it. So if there's no story reason for them to exist...
PrCs - OK, the 'better' ones, IMHO - certainly had story reasons to exist (unlike the MC-kludge PrCs, like the Mystic Theurge). The Purple Dragon Knight, for instance, was a position of renown linked to a specific part of the world, it'd've made a much better PrC than a sub-class, in 5e.
But, PrCs'd've been best used in products like SCAG, that were setting-specific, and offered players tie-ins to said setting. Campaign doesn't have anything to do with Cormyr, no worries someone'd play a PDK just to freak the guy who can't grok martial healing.
There's a reason why the "arcane warrior" half-caster has never gained much traction like the divine half-caster (paladin) and primal half-caster (ranger) have.
You mean much tractions as a full class. The classic elven fighter/magic-user has tremendous traction, it's present in 5e as not one but 2 sub-classes, and via MCing, and several feats and could be managed, nominally, by appending the soldier background to a wizard. It's pretty heavily supported.
As for the poster boys for superfluous classes that could be better done via MCing, the Pally & Ranger, they don't have traction, they have innertia. They'be been around since 0D&D.
The paladin? You can explain what a paladin is and what they do without making reference to being a warrior and that it casts divine spells.
Go for it.