Not mine but some common reasons
- Death
- Permanent ban
- Get rich enough and start to enjoy life
- Lack of easy bounties, good paid signature campaigns
- Need to focus on something else: learning to code, trade, invest or others that are not related to cryptocurrency
- Many people still visit the forum (as a guest) or logged-in account but don't make any post
Great example is Cryptotalk forum, that was once advertised here in signatures. Due to lack of bounty campaign (last bounty campaign on that forum ran more than half year ago) a number of users left that forum. After forum owners reduced the amount of post reward, great amount of user left it. Only spammers and those who have high ranked accounts and use forum for referral link promotion are left there.
They began with generous campaigns and as a global forum. Months later, they changed to a local forum that restrict many nations and it seems they have become a Russian forum.
Their campaign began with payment in
BTC but then in the shit token.