Bear in mind we have no answer for any of that yet. Researchers suggest that colder climates may have selected the best genes in humans in order to facilitate their survival. Sexual selection may have something to do with it as well. In general, northern races would hold intelligence and hardwork in high regard, so naturally these traits were seen as desirable in potential mates. Note that I'm not denying that physical appearance is important, but we can't rule out other traits that were also a crucial factor for our evolution. Furthermore, intelligent, creative, and hardworking people were more likely to survive in such harsh conditions and spread their genes.
So you have all of that. Now you ask me why southeastern Asians are smarter, on average, than Sub-Saharan Africans. It could be that the ancestors of southeastern Asians evolved in a different environment and eventually migrated to the current region they live in today. Maybe southeastern Asians have some percentage of White blood due to colonialism which is expected to increase the average IQ by a few points.
It could also be due to sexual selection. Maybe southeastern Asians placed a higher value on intelligence than Sub-Saharan Africans did. Sub-Saharan Africans historically killed some smart members of their tribes because they perceived their skills as some sort of diabolic magic. Education is also important. Southeastern Asia is better off than Sub-Saharan Africa in terms of school system and general education.
Hope this helps.
