ok once again:

it all goes down to the need for aggregation and self-protection.

If a criminally-minded individual decides to get busy he can either start his own thing or he can lean toward a pre-existing structure erected by experienced and like-minded crooks who've been doing what he's been craving to since before he was even born.

then geography and ethnicity come into play.

If the above mentioned wannabe happens to be an italian-lastnamed thug based along the "Italian Belt" (ny-nj-pa-ma-ri-ct-il-fl) then he's most likely to be dragged into the cosa nostra.

simply put this is how that thing is still up and running

If he's a non-italian caucasian based, let's say, in the upper-midwest then chances are that he might fall into the biker subculture or whatever german/scandinavian cabal society they may have over there.

and so forth...