[Jamie89]

Probably not a requirement but I think 2 things;
1: Thick people generally won't have many other opportunities to make money so a higher ratio of applicants will be stupid by default. The clever people will be busy working/studying etc and probably don't have the option to take 10 weeks off to sit in house with TV cameras.
2: Reality TV tends to cater towards a low common denominator, ie. thick people, who don't want to watch clever people and feel patronized.
(All massive generalisations, obviously there are exceptions)