It was also about maturity. By the late fifties the Mafia wasn't in street fights with any real rivals any longer so it didn't really need legions of street soldiers any more. It could make would be members take longer apprenticeships, which given increased federal attention and what some leaders saw as the declining quality of recruits, probably wasn't such a bad idea.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.