It was a weird source of pride among the old voters that no one was unanimous. The logic was something along the lines of player X (Babe, Willie Mays, Cy Young, etc) wasn't unanimous, so it would be insulting for a "lesser" player to be. No idea why initial guys weren't unanimous in the first place, but it's pretty stupid logic - guys would exclude players they knew were getting in just be make sure it wasn't 100% though. They cut back the number of voters and that has a lot to do with the things like PED users and specialists like DH and relief pitchers gaining ground.