It's a combination of things really. Part of it was not being able to discover new content (Google notwithstanding, since Google is good for finding things when you know what you want, while social media feeds you things you didn't even know you wanted). The other part was not being able to talk back or otherwise iterate on what people had published in a way that others could see it easily. Finally, it comes down to money... people wanted to make money and social media (and yes, I count Youtube among social media in its earliest incarnation) made it a lot easier to do that.

Despite this, people are departing existing aggregators and social media for SIGs to get around the spam, the screeching, and general annoyance... so we're probably going to see a more balanced network over the next decade... because making money isn't everything.