I think most popular reason would be failure to get into good paying signature campaign, or a reduce of profitable bounty campaigns. Or "earning using forum" in general.

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.

The reason I would leave Bitcointalk will be lack of interest in cryptocurrency (as a technology/opportunities it gives) or simply lack of free time (instead of reading forum, I would just read cryptocurrency related news webpages (i.e https://cointelegraph.com)).