Consider that "success in life" can mean different things to different people.

When I was in law school, my lefty peers were fussing about how we needed to recruit more black students. My response was "because you think black people haven't suffered enough?" But people who go to law school naturally tend to think that going to law school is the proper measure of success. Everyone wants to believe that the hoops they jumped through are the hoops that matter.

That aside...

Latin societies may not have had a LOT of social mobility, but they didn't have a color line or one-drop rule. So advancement was possible, even if only at the margin. Over generations, that could add up to your descendants being better off. Corruption and lawlessness don't change that fact -- they just change the mechanisms. Black Americans didn't have that incentive.

But as I said, this is something of a speculative just-so story. Maybe it's a factor, but Gato's observations on dependency culture and adverse selection seem to have as much or more explanatory power.