Why isn't it? It's their playground that they're letting everyone else play in. It's not like they can't set the bar wherever they want as far as quality goes.
There's a few reasons.
The biggest one is probably that "quality" is
immensely subjective. There is no one authority on what is "quality" and what isn't. If the Edition Wars have taught us anything, it should be that.

If WotC tries to appoint some Quality Czar to police all the D&D-compatible things produced in 5e, they are introducing artificial controls on what is "quality" that people are going to disagree with. This rules out potentially amazing products, and enforces a particular kind of very limited scope of what "quality" is.
That kills the innovation that can spring from an open market, reducing the value of the goods to whatever value others place in this person's (or team's) designation of what "quality" is, which erodes much of the point in having a supported marketplace to begin with.
The next biggest is likely that quality control is itself a quixotic goal. There has been no edition of D&D that has been perfectly balanced and perfectly well-executed after about a year of development. Arguably, shorter. If WotC can't guarantee that they themselves will produce reliably quality material (digital products, poor adventures, supplements with poor balance, ignored feats, etc.), then they sure as heck aren't going to be able to ensure quality from outside sources.
Which, again, is why I'm thinking that the "for pay" stuff is going to be directly licensed rather than an open OGL license. This lets them have oversight on projects and probably sets a fairly high bar for entry. Some guy banging away on a Word doc in his home likely won't be able to get his stuff in print for pay. Release it as a Netbook Of ... sure, no problem. But, if you want to get a book on the bookshelves sitting on the same shelf as the WOTC D&D books, I'm not really seeing a problem with letting WOTC be the gatekeepers here.
Absolutely nothing will be able to stop some fan from bypassing a license, charging for her material, and being entirely within the law. d20+modifiers vs. DC in a world of magic and elves is not in any part copyright or trademark. Hell, it's not even original WotC material -- d20s existed before D&D and so did magic and elves. They don't own the playround. They can't STOP you from playing on it. It's not theirs. They didn't build that. What they have control over is a brand, not a game.
Now, that brand is something they can leverage to the benefit of this goal of quality control. Imagine of all of
Dragon and
Dungeon magazines were Official D&D Content that fans submitted (not too different from the role they have occasionally played). Imagine if WotC owned a storefront and allowed would-be publishers to submit stuff to it that would become Official D&D material, vetted by their own dev team, and published only one or two each month.
There's a lot of value added to that brand, to that organization, to that expertise. Value that could even be worth some money (magainze subscriptions, DDI membership, a cut of the sales on each Official Product). That would work quite well, I imagine.
But the key is that they don't get the idea that this is the only way that fans are "allowed" to make and sell D&D-compatible material. They can control that quality in areas they do create (within reason, given the quixotic goal of quality control in the first place), but they can't control the quality of every D&D-compatible PDF that floats around on the internet. They can absolutely play a vital role of curation, of promotion, of encouragement, but they cannot hope to play a role of gatekeeper. Because ultimately, functionally, they don't control this game. We fans are the ones paying Mearls's rent, because we value what they make for our games. If we find more value outside, we'll go outside. We'll do what we did with the GSL and take our money and our passion and our fandom and go support great works that aren't in their gates, pay somebody else's rent for a while, somebody who maybe doesn't imagine that they get to define what "elf" or what "quality" means for everybody.