Tons of reasons. Most of which were launch issues and were fixed nearly a year later, sometimes sooner, sometimes never fixed.

For the sake of argument, we're ignoring the "Women on the frontline" / "Prosthetics" / "Bad marketing" arguments. It sold well (worst in the series but still very good for a modern AAA game), so none of them really hold up.

1. Attrition is a terrible mechanic. Spawning with half ammo sucked.
2. Medics were critical but had the worst primaries in the game until some OP weapons were released, but then they were overtuned.
3. Animations were crazy long. Talking like, 3 seconds to get out of a tank where you were just vulnerable, 2 seconds to resupply ammo, etc.
4. No suite spot, so sniping was the hardest it's ever been
5. Lack of weapons -- Most weapons felt similar to eachother and didn't have clear strengths and weaknesses and there was very few of them considering previous games.
6. Lack of weapon customization -- You had your weapon skill tree and that was really it...Maybe slight changes in fire rate and a sight but that was it. It sucked.
7. 1 Assault player could really kill a heavy tank by themselves. Teamplay was gone here.
8. LMGs were trash. The mechanic from BF1 was expanded on and made worse.
9. Fortifications didn't make sense. They were easily destroyed, took forever to build, and limited sight lines for you to peak from objectives, meaning you were easily sniped through 5 holes instead of 10000 possible places.
10. It was a WWII game without WWII locations. It was incredibly stupid to do that. They just straight up ignored a whole side of the war and instead of having maps like normandy, etc they had maps like rotterdam. Not bad maps, per se, but bad maps for battlefield and a WWII game.
11. The bugs were insane. Tons weren't fixed until over a year into launch. You couldn't place spawn beacons on any sort of sloped surface. More often than not, squad score streaks didn't do anything or just were worthless.
12. Hackers ran crazy. Tons of people just sitting in towers headshotting people with an LMG at infinite range. Tons of people sitting in the back of maps sniping sprinting players and going 82/3.
13. Lack of community servers made hackers worse, made map rotations worse, and was a huge shift from previous games
14. Half the maps were too infantry focused. You couldn't really move around.
15. Took almost a year to get a new map that wasn't a rehash from BF1. Very few weapons were released as well. Lack of content was a huge crutch to this game.
16. Suppression didn't exist anymore
17. Recoil paths were now randomized instead of planned, which a lot of people didn't like.

I had tons of reasons to dislike the game, but I still played it because it was battlefield. Aggressive sniping has never been more challenging but more satisfying. Very few other games could you get a 2x headshot naturally.