Most drug dealers make some "good" money for a short time but for the vast majority it never lasts. A few get it together and stop, but a lot use drugs themselves so smoke/snort/inject the profits away. Some get put away and some end up dead. It's the quick easy cash that draws them in, but easy come easy go for the most part.
I'm sure there are a few that rise up the ranks, but the major criminal drug barons would never be out on the corner dealing or anything like that in the first place.