Because a lot of blondes are fake hair color wise. Bleached hair and overly tanned skin, fake. Fake = attention seeking = slutty.
I'm not saying I agree. But I do know a lot of people who fill the stereotype perfectly. I'm blonde myself, and am far from a slut. But then again, my hair color is natural. All the slutty ones are fake blondes. Speaking from my own experiences of course.