Honestly it's all because of Zubatman. Wether /co/ came up with the idea all on its own or not all those years ago, the thread spawned a ton of little fics and the drawfags burst out of the woodwork for it. The following days request threads were littered with other DC hero's and their choice Pokemon, the content generated in those threads makes up the bulk of that type of fan art you'll see around these days.
Origin date is Aug, 31 2008
This was the first picture drawn.