Well, for one, we're talking about 5e as is, not 6e, so the Core 12 plus Artificer have the advantage of having been already printed, we're not going back in time to kill the Ranger, this has been about what to add to 5e.That's not necessarily unfair, but all base classes would have to clear that same bar.
Any bar the hot-mess of the Ranger, unfocused mundanity of the Fighter, vanished niche-protection of the Rogue, desperate forced mechanical differentiation of the Sorcerer, cultural specificity of the Druid/Monk/Barbarian, and/or the doable-with-MCing Ranger or Paladin could clear, the Psion and Warlord soar over with yards to spare, the Artificer probably clears with little difficulty, and the Shaman at least has a shot at with a good running start. Also in the running, all the various 'Gish' classes, because, damn, the 5e Ranger brings that bar down.
But, I mean, if your point is that only classes not already 'in the bag' (the PH) need to clear that bar, by all means, make a compelling case for that double-standard.
Second, a lot of the more niche classes can be expressed in relation to one of the core classes. The Gish is a Fighter and Wizard ... well, we have both of them, and they have subclasses expressing that character option. A Psychic Warrior is a Fighter that wields Psionic powers ... then haven't I just defined another Fighter Subclass?
Third, DnD is a genre unto itself, and the current suite of classes all have a history within DnD. Warlock is a term that has a slightly different meaning outside of DnD. Sure, a lot of that is built into the DnD Warlock, but so is a lot of other things, like Eldritch Blast.
And, while I remain a defender of 4e, it seems to get a bit of a bad rap in some spaces. So the classes that got attention and defining features in 4e, well, it's not necessarily good PR.
Finally, there is a lower bar when there's a lower number of classes, because you want to increase the diversity of class options. But as more and more classes are created, it does become a point of diminishing returns. The 20th class you add to the game is less interesting than the 7th.