Many people left because the forum is overrun by spammers.
We agree on the major reason is spam that is number 1 reason why old member feel it's a waste of time being here and it has been discussed gazillion times.
But there are many other small reasons which are causing seniors to leave this forum:
1. Lack of innovation 2. Lack of Management Involvement 3. Focus of discussion is diverted 4. bitcointalk is money making platformIf I may add, I think one of the small reasons is
the attitude of some users. Some users are so negative and easily judge someone else's ideas instead of helping the person improve his/her ideas.
When you are posting something that you learn or any insights that you have and desiring for a decent discussion to have on your thread but critiques will spam you of rude remarks - this will discourage the user to post again innovative ideas that will challenge the current status quo.