Because he was designated to be comic relief for an average movie?
Yeah, the only scenes in FF7 that even approach comic relief on Reno's part (that I can remember) are the Wutai bar, where the comedy comes from him merely not wanting to work on his vacation, and the scene in Gongaga Village, where he and Rude are discussing crushes. The comedy in that scene comes from Elena walking in on your eavesdropping, so again, Reno plays it pretty straight there.