I'm aware that this reason is sometimes trotted out, but I don't find it compelling.
NEWSFLASH: YOU DON'T HAVE TO find it compelling to have it be the truth.
Sucks, I know...and agree. But the fact is...you don't have to.
Part of the D&D brand is that it is a game of creativity and imagination, where you and your friends can tell your own fantasy stories.
Nice attempt to use my own words against me...A+...fact is, me and my friends, you and your friends...NONE of us matter. Will. It. Make. Money? That's what matters.
One True X (cosmology, dwarf, goblin, whatever) is counter to that brand identity. It is not a game about creating your own stories and imagining your own worlds at that point, it's a game about playing with other peoples' toys in certain pre-defined ways.
Which, 'nuthr newsflash, D&D has ALWAYS been!! Your elf is what? A D&D elf. Your "wizard" is what? A D&D wizard. Don't like it? Change it. As much as I despise the modern jargon, "Nuff sed."
I'm not allowed to imagine and create my own hellish afterlife, there's the Nine Hells, and if I play D&D, that's my hellish afterlife!
This is just so much hyperbole and, practically, hyperventilating that I don't even know what to say about it. Yes, that is exactly what they're saying. The D&D police is coming to
your house...no, not anyone's,
ONLY YOUR house to tell you what is "real" in the "fantasy role-playing game of gumdrop elves." Lookout! Here's comes the black-lycraed, but somehow incredibly pale-skinned, D&D gestapo. They are there, in the shadows...They're coming...no, not for any perceived "us"..just for
you. AHHHHHH!!!
So if you want to capitalize on the D&D brand, you don't want One True Way!
There is, literally, nothing here that is toting "one true way."...It's toting baseline...It's toting inclusiveness, which I had thought was your desire?...It's toting what WotC a.k.a. Hasbro WANTS [NOT Kamikaze Midget, HASBRO!!!]...based on what? Makin' mullah. Plain and simple.
I don't think One True Cosmology or One True Dwarf are the things that will give the game financial success. In fact, I think that because it works against the existing identity of the brand, it could actually hurt the bottom financial line (and the overall brand), if it's driven home too tightly.
And, interesting that you seem to know...what
is the "existing identity of the brand"? And, since you know,
how exactly, does it "works against" it?
I mean, conjecture about WotC's internal management policies in an exercise in reading tea leaves if ever there was one, so if they're getting pressure from higher-up that is distorting and harming the quality of the game, maybe there's not really much of an option for the designers. There's no real way for us out here to know, short of a leak.

If there's a
design reason (as Wyatt alluded to), maybe it's a good one?
You don't need a "leak." You don't need to read tea leaves [though I can if you like!

] You need common sense...and an acknowledgement of its [common senses'] existence. You need an acknowledgement that this is how businesses run...It's no secret. It's no magical formula. It's how things happen [businesses work]! You can accept it or cry "Why? Why?!" It matters not to me.