'cause they change it all back. It's difficult to understand why they would revert to what is clearly a lower technology level after making such a big leap forward, all within the span of a decade. It would be like having Apple II's in 1990, iPhone X's in 1993, and then back Performa PCs in 2000.
I once tried to come up with an explanation for a similar visual evolution/devolution as part of a crazy plan to reconcile the JJ Verse and the Prime Verse as the same universe. And, hey, it kinda worked. But it required some intense suspension of disbelief, and plenty of folks said it was a crazy idea at the time. (Essentially, I posited that Starfleet made a major technological breakthrough, implemented it throughout the fleet, but then a disastrous technical flaw revealed several years later forced them to roll everything back until the problem could be worked out in time for TMP.) Now something quite similar is canon.
This is, as always, not dispositive. Discovery could bend canon beyond the breaking point and still be a good television program. The root problem is I think it's a very bad television program. Because it is already bad, when it comes along and breaks canon, I'm not inclined to put in the mental work necessary to deal with it. And since Discovery absolutely delights in breaking canon, which breaks immersion for me as a viewer, they really are demanding a lot of mental effort from me.
This seems like an overly complicated solution to something that doesn't even need to be a problem. Perhaps this is a false equivalency, but the way I see it is...if you have to believe that the technology we see in Discovery has to downgrade itself to how it is portrayed in TOS...just to make them connected, then would you also need to think of a reason as to why the universe went from 3D to 2D between TOS and TAS.