Because the OGL says so. It's the rule that applies to all OGC. The OGL is the bible on OGC etiquette, because that's what the court is going to use if you get dragged into it over improper use of copyrighted materials. I also don't think for a second that WotC somehow "missed" that part of the license when they wrote it.
Besides, if it's not free for me to copy, modify, and distribute, then it shouldn't be free for 3rd-party publishers, either. Chances are "end users" do more for the game (from WotC's perspective; e.g. making them money) by inducting new players into the game (who go buy core rulebooks) than most 3rd-party publishers do filling a rack at the gaming store with cruft nobody wants or needs*.
Okay, okay; we can moot this argument. How about this: make up the UA SRD, and sell it for a penny. "Hey! They're publishers; it's okay, now!"
* Glittering generalization. May not apply in some cases. Restrictions may apply. Offer good while supplies last.