It changed far too many things for seemingly no reason. 3e at least withstood the initial backlash from the AD&D grognards because most systems were either tweaked or streamlined versions of existing systems so they'd fit the unified [d20 + mods vs. DC] model. Like saving throws, for example. They went from five fairly unintuitive categories ("why is staff/wand separate from spell?"), to three pretty straightforward ones.
Meanwhile, 4e added a ton of new terminology and a bucketload of systems that were brand new to most people. Yes, the At-Will/Encounter/Daily format was technically prototyped in the Tome of Battle, but not a lot of people used it. So if you change too much, the game doesn't feel the same, and people complain about it. Once 5e rolled in, it was back to the 3e format, but with further simplification and streamlining of systems. Instead of piles of bonuses and penalties, you get advantages and disadvantages. Instead of a myriad base and prestige classes, your class has archetypes you can choose to do different things, etc, etc.
I've said it a few times and I stick by it: it they had marketed 4e as Dungeons and Dragons Tactics, it wouldn't have gotten anywhere near the backlash it received. You could still play it as an RPG, but simply not making it the replacement to 3e would be enough.
