Why didn't Zeppo play the Oscar Shaw role?

I couldn't help noticing there was no reason for Zeppo not to play the male romantic lead. Was this the way it was on stage as well?

Zeppo would get bigger parts in Monkey Business and Horsefeathers, but he mostly hangs around the edges of their Broadway adaptations.

My best guess was that he was already losing interest in being in the act, and didn't really care how much stage or screen time he got. At the same time, the people behind the productions might not have thought enough of him to trust him with a bigger part.