Don't be silly Blood Omen, Steam is simply digital distribution. DLC started with MMO's and then found its way on to next gen consoles next.
Can you explain why Steam is shit?
Again, I think you are blaming the wrong thing. DLC is just another way of selling expansions, these were round long before. It was MMO's that really spawned the whole buy extra shit even though you already paid mantra.
Steam did not spark more DLC, think you are just linking the two because DLC has become more prevailant along with the rise of Steam.