Probably because blondes don't seem to brood and pout and get depressed and introspective like their darker-haired brethren might. So blondes are oblivious to whole nether-world of suffering and privation. They like bubbly fun inane activities (mostly chatter) and have no time for books, classical music, and preparing four-course meals. Why bother with filet mignon when you can just throw some Orville Redenbacher in the microwave.
That`s really funny, it seems to be so, that in many countries many guys mock about blond haired girls and women. Where I live, there are so many jokes about blond haired women and they all mean, this kind of women are more or less a little bit stupid or naive.
Its just a stereotype which has lead to so many jokes about it. I think that anybody can be smart if they want to and vice versa. It has nothing to do with race and hair color. I know a lot of very smart blond girls out there and one of them was my lawyer. She was a genius and very intelligent. I also went to school with a brunette girl that was ten times dumber then anybody I have ever known.
Because in the 1950's all dumb brunettes died their hair blond because they thought that was what guys wanted. Than all the dumb blond & dumb brunettes who died their hair blond held a convention. All the people who passed the convention said hey look at all those dumb blondes, and the phrase stuck. :\ I apologize on behalf of dumb brunettes.