Edit: uh, something happened with comment parenting, but anyway, yes, not every ad was mono-gendered, but in practice boys monopolized the C64s and TI-99/4As of the world.

It's chicken and egg situation - it was perceived as a "boy thing" so was marketed at boys. The 80's in the US & Canada weren't exactly Saudi Arabia, but toys were as gendered if not more so than they are today. And people marketing computers had no social agenda, they just took the path of least resistance.

It is, on some level, similar to the tragedy of the commons. Everyone made a decision that worked well for them and in the end the net effect was one that had a lot of negative impact.