Partly because beauty and brains is considered a rare combination.
Blonde girls are beautiful, so the other part is assumed like a default.
You know it would be unfair if a person has best of both the worlds !
And partly because of jealousy. Don't you think beauty and brains is deadly combination? Wouldn't you like to be handsome and intelligent?
But most of the people aren't both.
Actually, many of them including yours truly (unless the lights are dim and my mouth shut
) ), are neither.
Even this last remark is borrowed from someone from this forum, you see.
I can't say the same thing about Sardarji jokes.