Street gangs lack cohesiveness, order and secrecy. They want people to know they are in gang and wear it on their sleeves.... literally.

Both deal drugs but street gangs are much more closely connected to the drug activity in the streets which is why they are always getting killed over turf, the mafia dealt mainly with large quantities to other top echelons dealers behind the scenes and didn't get their hands dirty with low level drug deals on the block in plain sight. The mafia was like a wholesale clothing dealer and the street gangs were like store salesmen.

The mafia also had their hands in just about every major industry that they could make money from, both illegal money (via hijacking, drugs etc) and legal money ( I use legal loosely as the businesses themselves may have been legitimate but the mafia muscled in on them with intimidation and extortion). The mafia also had influence with people in high places, like judges, politicians and police whereas street gangs have little to no influence with important people and earn their money in the lowest level of practices like drugs, petit theft and credit card scams.