Don't think of UA as a true playtest. That's what their internal tests are for, with controls and full information and detailed feedback. UA is a glorified customer survey to get the temperature of the player base and see if they're going in the right direction. So they only put stuff in UA that they feel they can get useful information about.
Are they going to nerf a bunch of the over performing spells? That's gonna be hella unpopular, even if it's necessary, so they're not going to UA test it. Are they still making significant iterations on the encounter design tool based on internal testing? Then there's no reason to put it out for a UA popularity poll when it's still that unfinished, because any feedback would already be outdated by the time it was processed.
The main reason for doing this public playtest is to provide cover against any criticism they didn't actually fix any but the most superficial of problems with the edition.
WotC's main objective here isn't to actually take any risks you need to take to actually fix anything.
WotC's main objective here is to be able to keep milking their cash cow.
How make everybody purchase all their 5E books all over again without making people mad?
By making a public playtest. This way they can scrap any risky ideas since they will invariably fail to reach a high degree of satisfaction, since people can't handle having things taken away from them, even when it is for the greater good.
Had D&D Next come out right out of the blue, people would be mad it mostly contains niggling small tweaks that mostly serve to invalidate the 2014 content without actually fixing much.
Now WotC can tell us "this is what you wanted".
Contrast the release of 3.5 with the release of 5E.
In the first case, WotC promised every problem with 3E would be fixed. 3.5E is almost the exact same game as 3E. Yes, it's not worse, but all the systemic failures of the edition remain.
Now consider 5E. Here WotC was with their back to the wall. They really wanted to fix things, after the debacle that was 4E. They did not say silly things like "unless 70% of you agree to scrap the 4E Warlord we will keep it".
This resulted in actual progress. 5E
actually does fix many of the things 3E was accused of.
It could not have happened with today's WotC. Today's WotC just want the money to keep flowing in. They have no incentive to risk angering their customer base even if that would actually make for a better game.
But this was exactly what was needed in order to create 5E in the first place.
I predict that the 2024 edition will be remembered as a new 3.5. Everybody will move over to it, simply because WotC has made sure to tweak an annoyingly large number of things making the statement "you can keep using your old books" suitably hollow, but it will not fix much of anything.