Up until about 5 years ago, the Diablo II community was still bustling as much as ever. It once boasted an average of 20,000 online at any given time, while it's roughly 8,000 now.
#1. Spamming Ad-Bots
By the time patch 1.13 came around in 2011, the game very quickly became infested with spamming ad-bots for websites, like Lewt, that sold items and leveling services for money. These bots would join games, and within two seconds, fill the screen with text, then leave the game. This became intolerably annoying, especially when between 2014-2016, it was so bad that they would join constantly back-to-back, sometimes simultaneously. If you made a new game, you'd have to expect it to fill with ad-bots for the first minute or so.
Not only was it annoying to have your screen spammed, but the bots joining and leaving fast were constantly triggering an old bug that caused a game to designate as full when it wasn't. So you would leave a public game, expecting to join it on a different character, and be blocked by a "Game is Full" screen when it wasn't actually full.
This went on for too long. Over time, the servers became less and less active. Patch 1.14 in 2016 finally took care of the issue, forcing spam bots to relegate to overhead spam instead of chat spam. It was too late, however. By 2016, the community was halved. It's likely that most players who quit still aren't aware that the issue was fixed.
#2. Botting
This is a hot-button debate in the communities, because botting is so popular and they DO provide some benefits to the in-game community by providing consistent runs to follow and lowering item prices by saturating the market with them. I am one to think that the bots cause more harm than good, however.
For one, cheaper items is a double-edged sword. Players who don't use bots get less items and get less FOR the good items they do find, so they get a smaller share of wealth overall. It's like trying to operate a small shop beside a Wal-Mart. It's not fair competition and therefor results in unfair wealth distribution. The wealth distribution between botters and legit players means that botters hold all of the best Tier 1 gear and, at best, legit players get 2nd Tier gear for dirt cheap. Sure, you can get a Shako for next to nothing, but try getting a good 2/20 circlet and 45-life skillers without botting. The excitement of finding a valuable item and getting a fat payday for it is mostly gone, because bots have saturated the market to the point where demand is low. That said, the server economy has devolved into "Bot or be poor". There are a lot of people who don't care for that situation, including me, and have moved on to just let the botters duel each other.
#3. Modding
What little interest I would still have in playing on B.Net is smothered by the reality that the PvP community is infested by GMod. For those that don't know, it's an all-in-one "hack" that provides fastmod, farcast, auto-tp, auto-Save&Exit (Chikin), Auto-Rejuv, and more. Dueling is a complete farce and waste of time when you encounter these modders in just about every game you join.
Every veteran player will tell you; Diablo II on B.Net is a PvP game. It thrives on healthy and fair PvP. Combine the botters' gear advantages with a GMod infestation and you have a game that legit players have no interest in playing anymore, aside from holding out hope that they can still enjoy their old favorite game some other way. As I said earlier, the modders and botters can duel each other, we found other things to do.
#4. PlugY and Path of Diablo
PlugY Mod gave the legit grinders like myself a way to enjoy this classic game without having to put up with any of the issues with B.Net. It's our Sanctuary from all of that shenanigans. We get infinite shared-stash space, so the end-game possibilities are endless. We also get ladder runewords, Ubers/Clone D, and infinite respec's on skills and stats.
I do not care for mods that alter the game beyond that myself, however, Path of Diablo has become very popular the last couple of years, with DBrunksi himself streaming it regularly now.
No doubt, these two mods have provided an excellent outlet for the players who are tired of competing with the infestation of modders and botters on B.Net. No doubt as well that the Diablo II community on B.Net is dying rapidly as a result.
...and maybe it should.
#5. Diablo III
I don't like to give much credit to the farce that is Diablo III, but I'd be deluded if I didn't acknowledge that it did take a chunk of the Diablo II fanbase with it. Many came back, but a decent amount did not.