Well, it is simple, in 2010s the YouTube was not mainstream, it was mostly followed by the younger generation. It is where they found content they wanted. This is the time when we see YouTube personalities take off. That was the period when Felicia Day, Angry Joe, TotalBiscuit, Jim Sterling, Yog Cast, etc. popped up. I remember this time as when Machinima was the biggest thing on YouTube and multiple YouTube personalities made mini TV like series. Basically, this was the Wild West period of YouTube. To make it you needed knowledge of hardware and software to record videos. and both were not available easily or could be used easily.
That period ended. The end came as the YouTube stars came of age. 1st the major companies saw that cable subscriptions are going down and there is money to be made on YouTube so they started to move into YouTube. Now it was no longer a group of small creators financing their software and equipment, it was companies that have been recording content for years with large budgets throwing their weight around. This allowed them to push easily against the small creators. Those like Rooster Teeth survived because by that time they became big themselves.
The second reason that period ended, because everybody could become a YouTube creator, it is a low barrier for entry. Software and hardware are abundant and easy to use, and so this created a deluge of content of people who are trying to replicate the YouTube creator from 2010s. As the result they drowning out each other.