While beauty is beauty, and there are beautiful women with every different hair color, men NOTICE blondes immediately, and with all of the options women have to enhance their appearance, a blonde girl with a plain face who takes advantage of makeup & such can look "smashing," while other hair colors cannot accomplish the same thing as readily.
To illustrate, if Norma Jean Baker had kept her brunette hair, no one would have ever heard of her.
And obviously women believe that blonde hair is attractive to men, as so many of them bleach their hair, some with dreadful effect.