There are a lot of possible reasons.
Perhaps there's not enough promotion of this website, on the Internet or by members/guests here. This site seems like it reached its peak several years ago. Maybe there were less Islamic forums around then, which promoted traffic to this one. For whatever reason, there might have been a lot more people suddenly interested in the Qur'an alone approach, or a lot of older members still active, or coming back, at that time. Maybe the community environment and atmosphere (friendly vs toxic, for example) was different than it is now. All communities go through stages that influence population. Users could have simply made more new posts than we do now. Like good logic said, on this forum we may have talked everything to death. At this time, we have 396,996 posts in 21,539 topics. There may be no reason to have the level of discussion that we once did.
Perhaps in the past, guests were more likely to make accounts to interact with the community. Instead they lurk. Maybe many of them are simply not ready to make an account, or don't feel that they have anything valuable to contribute to the forum. I know we've had people introduce themselves here who have been lurking for years.
Of course, there are also the obvious reasons.
Peoples' lives get busy. They can easily forget about this forum, or simply find that posting/reading here isn't benefiting them anymore, and that they need to focus on other activities in their lives. I know we've had posts from members who official say their goodbyes. Some of them may come back at a better time. Some of them don't.
Some former members no longer believe. We know this. Some have come back saying they don't believe in God anymore, that they no longer believe in the Qur'an/the Qur'an alone, etc. In which case, why would they still be here? They've moved on, as other members (the state of their belief aside) have.
