Guess it became too cumbersome and noticeable for 250 men to squeeze into a cafe or social club. It would also be much harder to control the outcome. The Underboss is always selected by the Boss. Today, the capos elect him. In reality, of course, he's appointed by the Boss since capos are selected and serve at the pleasure of the Boss.
Soldiers are the lowest ranked members.
At the turn of the century, only full blooded Italian males could be inducted. But today, membership can be attained by men whose fathers are of Italian heritage. Today, there are said to be some who have not taken part in a murder, but they are few and far between. During a Mafia War in 1930, standards were lowered and friends of friends were inducted. When the killings ended, unhappiness with a number of these unproven made men became a major issue.