Yahoo messageboards are relics of a bygone era. The financial content has always been a front to market to a certain demographic.
The basic problem is that messageboards exist as a medium for anonymous social interaction with strangers. With all of the nutjobs lurking on these boards and the lack of privacy controls, you have to be very careful about what you post. It's not a good user experience, and if people are actively discouraged from sharing personal information that can be used for targeted ads, it's a bad business model as well.
Most people interact through sites like Facebook and LinkedIn now. It's as simple as that.