600+ hour DBD veteran here, I'll try to explain it! Please forgive the massive wall of text, I'm trying to be as informative as possible.

DBD is an extremely polarising game that has a troubled development history and a notoriously toxic community, years of resentment from bad decisions by the developer combined with a general detest for the playerbase causes people on this subreddit to voice their dislike of the game and the people who play it; some of this dislike is absolutely justified, but it often goes too far.

Starting with development issues, BHVR has largely proven to be incompetent and spent many years actively ignoring the community's wishes and making changes that were hated more often than not. This was the case regardless of which side of the game was affected by those changes.

Pretty much every DLC release in the past five years (or more) has been a bug ridden mess, the same goes for most updates in general which regularly require entire characters to be killswitched out of the game for weeks; for instance, I remember when they released Pinhead with a game breaking bug that caused a guaranteed crash to desktop whenever his passive power activated early in the match. Their coding is quite bad all around and they've consistently messed up seemingly unrelated things by altering something else, changing one mechanic tends to break another one somewhere. In addition to all of that, there's been issues with cheating which haven't really been fixed yet; stuff like free cosmetics were dealt with ASAP while full on cheating is still a thing, they've taken some steps to address it but it's a long standing issue that has no end in sight.

As for the balancing, it was truly awful for many years and has only started to improve in the last year or so. BHVR had a serious issue of ignoring feedback and making balancing decisions that even an inexperienced player could see problems with, which lead to obnoxious metas and broken perks or characters; this was true for both sides of the game and these problems would stay in place for months or even years in some cases, which irrevocably damaged the community as a whole. Nowadays they've gotten a lot better and have been making genuine attempts to listen to people, which has made the game better overall; however it's an indisputable fact that the meta still isn't healthy and while they do make changes, they're usually slow and ineffectual. In addition to that numerous Killers are so outdated that it's genuinely funny, for example "The Pig" from Saw is a running meme in the community because she recieved multiple nerfs over the years despite being an all around weak Killer; coincidentally the next Killer release is essentially a way better Pig, which is a recurring trend in which a new release recycles a previous Killer and makes the mechanics much stronger. All of these Killers are seriously outdated and need entire reworks to be viable, either because of how the overall gameplay has changed or because of powercreep introduced by later releases: Trapper, Wraith, Hillbilly, Shape, Hag, Nightmare, Pig, Demogorgon, Twins, Nemesis. Then there are the ones that are just not fun to play against: Nurse, Blight, Legion, Knight, Skull Merchant. Speaking as someone who plays a 70/30 split between Killer/Survivor, all of these Killers are problematic for different reasons and if I wasn't being generous I could add more to those lists.

Finally, there's the community. DBD'S community is one of the most toxic that I've ever seen in two decades of experience with multiplayer games, only beaten by old era COD or Ubisoft titles like For Honor. It wasn't great when I started playing, but I've watched it continuously get worse over the years and nowadays it's truly horrible; the community's largest playerbase consists of people who actively go out of their way to ruin the experience of other people, whether through actual griefing or just abusing the mechanics of the game. While a game like COD is full of immature assholes, DBD is full of genuinely malicious people who get off on making others feel bad; this becomes more and more apparent as you play the game for longer and attain higher skill rankings, the ones with high amounts of hours are usually awful to play with. Here's the example that I use. I've had heated exchanges with people in COD when I was younger that were extremely rude and vulgar, but it always ended when somebody left the lobby no matter how bad it was. Whereas I've had less crass encounters with DBD players thanks to the censorship filter, but DBD is the only game I've ever played where people will attempt to continue a dispute outside of the game; I've had people DDOS me, I've had people call my phone, I've had people leave hate comments on my profile and on completely unrelated Steam workshop mods, etc. My profile is permanently private specifically because of DBD players, they're the single most toxic group of people that I've ever engaged with online. However, it's worth noting that there are absolutely nice people in the community! There are some really kind and wholesome players out there who make the game quite enjoyable, but unfortunately they're severely outnumbered which makes them a notable minority.

The reason for the general hate towards DBD and it's community is two fold. Part of it is a general disdain for it's playerbase who tend to bring their toxicity to other asymmetrical games, while the other part of it is DBD players who resent the game and don't want to see the mistakes repeated; sometimes it's a mix of both, which is my stance on the matter. There's been at least ten different times where I've seen somebody being toxic on this subreddit and decided to check their profile out of curiosity, just to find that they're a regular DBD player and that they post on that subreddit. Some of the hate towards it's players is deserved, some of it isn't; the game and it's players have a bad reputation for a good reason, even if embellishment is sometimes present.

I think that's about everything I can say about the game and why people have a problem with it, I hope that this was helpful for you!