I don't know for sure, but my guess is that it went like this at the pitch meeting:
Pitcher: Sir, we want to do a cheesy, goofy superhero show. We've spoken to the folks at DC Comics and they agreed to let us use one of their serious characters and turn him into a lame mockery of his former self. Since Superman's already been done on TV, we were thinking Green Lantern, or maybe Aquaman since he's already really lame.
Pitchee: What will it cost?
Pitcher: A hillion gajillion dollars, because those characters have superpowers and would require a lot of special effects.
Pitchee: Ugh. No way. Isn't there a DC Comics superhero with no superpowers who we could do without any special effects to speak of on a really cheap budget?
Pitcher: Well, sir... there's Batman, I guess, but he's very dark and tortured with no comic relief to speak of and wouldn't fit the show at all, and beyond that he's the coolest superhero in the history of the world and we'd hate to ruin him for all the loyal fans out there.
Pitchee: But what would it cost?
Pitcher: Very little.
Pitchee: It's Batman. You're doing Batman.
Pitcher: But sir--
Pitchee: SILENCE, plebian! Batman it is.