I'm sympathetic to two main concerns I've seen.
- The creation rules are not newbie friendly, and they wanted to target newbies more. This makes some sense to me, and is something I can get behind in principle. This still ruins how I use D&D, but I'm definitely a pretty elite user. I do wish WotC wasn't afraid of teaching DMs in their books, but I kind of get it. Here's me hoping for a revised system in a future book (which may also include species / class / subclass /etc. design guideance!).
- The creation rules caused a lot of unfounded griping. No monster creation system can be perfect, and there will always be exceptions to the guidelines, but the main chatter around the creation system was that "it's bad," either because it wasn't perfect, or because people didn't really understand how to use it. Wizards decided that no system was better than a system that was attracting so much negative attention, or couldn't figure out how to present the system in a way that wouldn't give this sort of false impression, and so they just dropped it. This situation is a bit more dire, because it implies that we may never see an updated monster creation system.
But, the monster building system was critical to how I was running D&D games and it worked very well for me (in combo with Xanathar's encounter building rules). So it's one of the big things that makes me unhappy to run 5.24. Until it's fixed, it's gonna be a problem for me, since I can't run games in D&D the way I want without it. Whatever the reason for keeping it out, I want to see it put back in, as soon as possible.