Black Americans are mostly native born. Asian Americans are mostly immigrants or just one or two generations removed from immigrants.
Immigrants from Asia can pretty much only get a visa through some kind of work visa. That means they are pre-selected for having a highly-demanded skill set before they can even enter the country. It also takes decades for them to qualify for permanent residence, so they need to be employed in that field for the entire time. It is almost guaranteed that these people will be financially successful because someone who is employed for decades in a field that can qualify for one of these visas will be financially successful. Anyone who isn't will not legally be allowed in the country.
US law pretty much guarantees that Asians on average will be more financially successful than average because they are much more likely to be here on a visa that requires financial success.