It's the society in general that has become more relaxed when it comes to how people dress. It's a product of egalitarianism that has prevailed in the West in the last few decades or so. Even doctors, university professors and other rich and respected people walk around in jeans and sweaters, mafia/OC just moved with times, like it always did.

They're a secret society after all, they have no reasons to stand out and attract attention. It doesn't have anything to do with copying "gangbangers", look at pictures of mafiosi arrested in Italy, they look just like these Bonnano guys, and they probably don't give a shit about USA gangs. Tracksuits are the uniforms of the 21st century OC, at least for the ordinary 'soldiers'.