I'll preface this with the fact that I played back at the very beginning, got very high up, and then quit for a while, and recently have come back. I'll also say that there was a point in my life where I was managing about 25 teams on this site and similar sites (I was really bored in college). I also work on the internet for a living. So I've got a pretty broad perspective I think.
all of these games are going through similar problems, and here's my view on the 3 reasons why (I'm sure the creators are aware of all off these):
1) The forum moderators aren't super polished. Sorry guys, and I'm probably going to get in trouble for this, but they've historically not been great at creating or encouraging conversation.
2) The market is out of whack, and certain features aren't well-balanced. This plagues every app/game at some point, but people get too good at figuring out whether there's more value in, for example, drafting or taking players from the transfer list, and then that swings when players leave the game, and eventually parts of the game can become dead.
3) The ability to compete with apps, without BECOMING an app. People love apps. They will swipe endlessly on their farms, to use the old euphemism, burn out on a game in a week, and move on. This isn't that kind of game - it takes patience, the long game, you check in for a few minutes in a day, and then come back on game day. At the same time, there's no BB mobile presence. There's not a good mobile site, there's not an easy way to update your lineup from your phone, on the go, while on the subway.
I'd love to see a BB app that is just a better interface for the web version of the game. I think that would be great for its growth. Don't drasatically "appify" the game, just give users a way to deal with it 'on the go.' I think it's a way to keep users engaged, remembering to update rosters, etc. Especially with the site blocked on a lot of work filters.