Because abilities correlate across wide swaths of personality traits.
A group of highly skilled competent people working on a shared highly specialized project will more likely than not be similar in other ways - enough so to establish common behavior patterns which become a shared culture that then gets adopted as a default mode of relation for efficiency.
Extrapolate that dynamic up to an entire sector and you find that certain combinations produce better results. Those then become popular and spread until they are all quite similar and stable.
Voila, a monoculture is born.
