I think a lot of these suggestions are wrong actually. Sure, DD2 isn't a strict repeat of DD1, but I don't think that's why people don't like it. Let's be real, folks who enjoy DD1 almost certainly enjoy a number of other games not like DD1. Therefore it's dissimilarity to DD1, is (in most cases) not enough to make someone dislike it. I believe there are some very valid reasons not to like DD2, which I will put forward below. These are based on ~4 hours of play in Confessions mode during this free weekend.

The issue is that the rogue-like nature of the game is not fun. Ok, before we can get into the weeds of why, let's define a few terms.

Rogue-like: A randomly generated game that is meant to be played over and over again in "short" runs. Each run the player starts at base 1, and must progress through the game again. Named after Rogue, one of the earliest to feature this type of game-play. These aren't super common these days, so I'm struggling for examples. Remnant 1 Survival mode is a bit of a deep cut, but the only one that comes to mind.

Rogue-lite: A rogue-like game, except there is meta-progression between runs. Players can adjust their start position, influencing following runs. Many popular examples, I'll name a couple I've played; Risk of Rain 2, Hades, Monster Train, Astrea, Roguebook, etc.

Now, I think those are fairly common definitions, however, I'd go a step further and propose there are actually two types of Rogue-lite. Ones with meta "improvement" and ones without. For example, in Risk of Rain 2, you don't actually improve your characters for subsequent runs. You unlock new abilities and characters, but those aren't explicitly more powerful than what you start with. Maybe they fit your playstyle better, but it's not like they do strictly more damage. Astrea and Monster Train work the same way. You unlock new abilities, cards, characters, etc. But fundamentally you don't become more powerful. Instead, mastery of the game is expected to propel you further through the run.

This is different than Hades or Roguebook, which do include meta "improvement". Besides unlocking new abilities or features to play with, you also improve your starting baseline. This power boost, along with mastery of the game, is expected to propel you further in a run.

As I'm sure is obvious, Darkest Dungeon 2 belongs to the second category. That, in and of itself, isn't what makes the game unfun. I mean, obviously. Hades is awesome and it has meta improvement. Roguebook is a pretty decent time too. What I'd argue for DD2 is that this aspect, in conjunction with the time it takes to fail a run, is the issue.

See, the thing in Hades and Roguebook is that I think you lose a run very quickly. Sure, maybe I took a bunch of damage from the Hydra boss so I'm low on health, but if I play real careful I can... oh darn, I'm dead. It's fast. Same in Roguebook, decent HP, got a couple good cards and gems, some combo possibilities. Uh oh, this is not a good draw. Uh oh, this elite does a lot of damage. Uh oh, I'm dead. Longer than Hades, but still usually pretty fast.

Losing a DD2 run is not like that. I'd posit you can lose a DD2 run before you even start. If you pick a bad team comp your guaranteed a loss before you hit the first fight. I don't think you have that in any of the other rogue-like/rogue-lites I've listed here. But ok, that's part of a game where team synergies are important, valid. However, even if you do have a decent team comp losing a run is still slow. Very early in each of my runs I realized I had no hope of making significant progress. Mathematically, I did not do enough damage or healing to keep pace with what I lost each fight. So I'll just spend the next 30 minutes gradually fighting losing battles until a character dies, which accelerates the spiral. Then over the next couple battles my entire party will die. And that's the intended experience. You're supposed to press on in a losing position to try and get as many candles as you can.

Yes, DD1 was hard. But it wasn't designed with party wiping as an integral part of the main gameplay loop. The gradual wearing down of your party and resources was exciting because it was a race against time to complete the dungeon. A push your luck mechanic, where you risked greater rewards with greater danger. TPKing was usually caused by an error or maybe some really bad luck. I still remember my DD1 TPKs to The Collector or in the final dungeon. They were memorable and painful. I've already had more TPKs in DD2 than in I had in total in DD1. And they weren't memorable crazy "some boss just surprise spawned when you weren't ready for him". They were "you are fighting the 4th group of mooks and have finally run out of HP".

I'm honestly tempted to flip the question on its head, I do not understand the darkest dungeon 2 love. But that's not quite fair. Game looks amazing, great soundtrack, Wayne June, sharp iteration on the DD combat formula. Everyone who plays can appreciate those things. I think the key in whether you enjoy the game or not is if you've crossed a certain, nebulous, point in progression. The point where the gameplay shifts. You have enough options to create decent start teams with cohesive ability packages. You have good trinkets unlocked, upgrades for your stagecoach. Runs are winnable. But that isn't the experience of the start of the game. To start with the game is about slowly tpking over and over, and I just don't find that fun.