The Buddha implies that the majority of people are, for lack of a better word, stupid.
Only a few have little dust in their eyes.
I’ve met intelligent people. But I also recognize that there are an increasingly large number of stupid people surrounding me.
As for the causes ... seriously? Look to genetics. If you asked “where does beauty come from?” - you look to genetics and human breeding.
Beauty and intelligence don’t always go together. All the same, look at dogs ... we have dumb breeds and intelligent breeds.
A German Shepard is intelligent but also unpredictable and viscious.
A Border Colly is very intelligent and loyal and not viscious.
People have done the same thing throughout history. The whole establishment of arranged marriage is a breeding program.
The Buddha had it all. Supreme intelligence and looks - the outcome of hundreds to thousands of years of breeding for a number of things - strength, bravery, looks, intelligence, loyalty, etcetera.
Keep in mind that Ashkenazic Jews (like DNS for example) have an IQ across all members of this group which far exceeds the norm for any other group of people in any given nation.
It’s all about breeding. It’s not about kamma. Unless you say, “well, this person will show intelligence - and then what being inhabits this body? What did that being do in a past life to inherent such a well bred birth?”
The questions become endless at this point.

Pondera