After a minimum level of IQ, there have been big men successful in the nba. Because they are big and athletic. Higher IQ, low athleticism athletes also don't get drafted. Many great college basketball point guards have fit that bill.

I don't think Russell Westbrook was a higher IQ point guard than Billy Donovan but he dominated for a time in the NBA and Billy got cut after one season, both for athletic reasons. Westbrook could be taught and learn the game good enough with his athleticism but Donovan couldn't become athletic enough.

Generally, to be in the nba, you have both, OR have the minimum requisite in one of the areas and be the elite of the elite in the other area.