My guess:so how did Mike Mearls end up as "the guy" with 4e?
1) His hiring by WOTC was seen as a significant event. He'd already made a splash in the d20 world, so maybe it was assumed he'd strongly influence the new rules.
2) He was posting controversial proto-4E stuff like rust monsters that got roundly panned on ENWorld on the lead-up to 4E's release, so people probably associate him with it.
Monte Cook sort of unofficially got the credit for 3E in the public eye as author of the DMG, and a cult sort of grew up around him. Maybe people like to fixate on someone.