There were 3.5 players who had not yet embraced the idea of classes like the Warlock, who was unchained from the Vancian model of spellcasting, instead having unlimited use powers, nor the "encounter-based" design of the Tome of Battle. The WotC developers had decided that balancing a game around spell slots and set numbers of encounters was problematic because many groups did not run their games that way.
(And still don't, which is a recurring issue with 5e).
That 4e was going full steam ahead with both this new paradigm, when you had old school gamers perfectly content with resource management and slow grindy adventuring days really stuck in the craws of a lot of people.
See, there was a huge disconnect between the way people talked about 3.5 on forums, and the way it was being played in home games. On forums, you had tier lists, a solved metagame, and complaints about "why not Tippyverse" or "why not scry and die" or "why not God Wizard" or "why not Mind Redacted" or "why not 5 minute work day"?
In home games, you had people playing PHB1 Fighters and not having any problems.
WotC, of course, paid the most attention to the vocal part of the fanbase, who were constantly pointing out the flaws of 3.5, griping about the bloat, and showcasing ridiculous builds.
So 4e was made to address those concerns, giving us classes that were balanced against one another (more or less), shiny new options, sticky Fighters, and a massively rebalanced magic system along with toned down magic items with more conservative benefits.
And well, I know I was a DM for one of those groups that was not yet ready for the changeover. Eventually we were, and we had a lot of fun with the new game. But just as there are people who consider 1st Edition to be "true" AD&D, and there are people who rejected 3e as "that horrible WotC edition", there were always going to be people who rejected the fancy polish of 4e. "Where's my Gnomes? Why are Dragonpeople in the PHB?", they cried, despite the fact that a given D&D player has seen maybe 5 Gnome PC's in their gaming career.
The irony is, if D&D had remained the only game in town, I think more people would have accepted it, just as many people have accepted 5e. But...Paizo made their move, and gave people who wanted a "fixed" 3.5 a new game with new content and shiny new art, and from that moment, 4e was doomed.
Never no mind that Pathfinder 1e ended up just as bloated and ridiculous as 3.5 (though admittedly in different ways) by the end, lol.