It had a lot to do with Warner Bros wanting to use more IP in their movies. When the execs found out that most Vertigo titles were creator owned, they had their lawyers rework the contracts so WB could retain ownership of the works, which caused a mass exodus of talent. Dark Horse and Image picked up most of the talent.
It’s the same as how most airlines operate. The first and business class sections make up the majority of the profit, even though they are just a small fraction of the passengers.