Because boys would just wreck them. Duh...
When my sons were in preschool their girl friends would come over with their Barbie cases and Barbie dolls and tons of Barbie clothes and the kids would all sit around dressing and undressing and having them talk to each other - lots of imagination. So I got them their own Barbie dolls and some clothes which they liked for a while .... until the day I walked into the kitchen and one son was cutting off Barbie's hair and then bended her into the shape of a gun. And Barbie became a weapon. That was the end of the Barbie phase.
Boys would have a great use for them: pellet gun targets.