But even so, it begs the question, why shouldn't it?
Perhaps it should; perhaps the franchise needs a reboot. But IMO not in that way.
A core
more generic,
more mythological might have been appropriate. Less arbitrary. Clerics seem pretty arbitrary and specific, much as the newcomers dragonborn and eladrin do. I wouldn't shed a tear to see "clerics" get a rename and an overhaul, or see Paladins renamed Knight or Crusader or something, or Ranger turned into Scout.
Less D&Disms, not more, would have been the way to go, IMO.
WHY?
Because so far as I can see, people USE d&d as a sort of fantasy worldbuilding kit. Specific, D&Dism stuff in the core implied setting just alienates the game from that (and by core implied setting, I mean contents of the PHB everyone is assumed to use). Down the track, if you want to go Cthulhu with your mind flayers and aboleth and Elder Elemental God, or steampunk with your warforged and artificers and magitech, or all Wire Fu with physics-defying martial arts and ninjas and kappas, then that's a personal decision. Right next to you is a person who doesn't want that. The implied setting should cater to both of you, at least out of the gate.
The default is (or was, or should be) D&D's Tolkienismesque implied setting, because the Professor kind of hit a nerve there, putting all this rich mythology together (yes, I know he wasn't the first but he was the most well known) and D&D profited immensely from copying that (even if it were via a circumspect route, as Gygax swears that Tolkien wasn't the source). Dragonborn warlords and eladrin blinkathons straight out of the gate, non-optional, compromise that, IMO. There's more "what the heck" there than rich mythology.
Why don't we have that? I expect it's related to selling books and miniatures. If you have an all-inclusive core, continually piling on the splat as non-optional, people will buy everything. May as well get them used to the idea by putting in arbitrary, random, thematically mixed stuff in the first PHB.
Or maybe they'll become fed up, and buy nothing.