I think that the main reason why Bitcointalk is losing it's popularity is that Bitcoin went mainstream and other businesses/platforms used this opportunity very well.

1. Websites like Cointelegraph, Coindesk and others try to write and publish news as soon as possible, these websites are a great source of crypto-related news, they are the ones who search the information all the time and then show it to their audience.
2. Websites like Stackoverflow have an audience of developers and enthusiasts who are more likely to help each other and share the knowledge.
3. Reddit is one of the most popular social news website and it's not oriented on one task but there is a discussion and boards on almost every theme/subject and people prefer to use it because for that and also for the nice UI.

There are some other reasons to say too along with them. Also, at the same time, don't want to look rude but Theymos doesn't really take care of this forum. I don't judge it, I really appreciate bitcointalk and its role in my life, it completely changed my life for better but truth is truth, this forum looks like abandoned and the major life-giving power is the signature campaign at the moment.

Bitcointalk needs serious change, implementation of new features to make the website look like a comfortable all-in-one service.