Honestly, they changed just enough to sell more books. Its not really backwards compatible at all, because it requires all sorts of dm futsing to decide how each thing works together.
Beyond that, they failed to address any real issue and just complicated things. Play above level 6 is still a joke. They once again gave zero thought to it. Further more there basic inability to even proof read what they did write. The problem spells are even worse or simply no better. Why is fireball still better than many level 5 spells? Oh yea, they don't care about play at lvl 9 because it would take a mediocum of effort to worry about the game past lvl 6.
People talk about updates and erratas to fix the glaring problems, but they had 11 years to fix basic issues in 5.0 and did nothing. Why does ANYONE think they'll be better about 5.5. The dmg wont fix anything, it'll continue the trend of giving the gm MORE work with wierdly worded rules and all sorts of wierd situations.
Then we can prepare for redos of all the other books. They'll just remake them for 5.5, because its low effort.
Lastly? Because dnd is supposed to be a table top game, not a video game. They are heavily gamifying everything, making it more like bg3 and optimized for dndbeyond. They somehow removed even more of the social/exploration pillars, because thats not fighting and rolling dice. Honestly, I'm afraid of what 6.0 would be, because they're taking the worst parts of 4.0 and promoting them. The current creators of dnd are ruining it. This isn't just wotc corporate, this is the people who do dnd making this garbage. Lets not pretend they aren't doing it on purpose and just "blame corporate".