I agree, there seems to be an unnecessary double standard at play there. It would be interesting to see where it comes from.
I think we all know where it comes from. There has been a preconception in the gaming world that "girlz dun pley gems" since the dawn of time itself. This probably stems from the fact that most early games were action-adventure games and probably didn't appeal to many females at the time, although we all know that times have changed since then but I'm sure the preconception still exists in many game developers' minds. I would also say it has something to do with how females are expected to behave in our society: nice non-sexual creatures for males to play with when we desire, get in the kitchen and make food, and raise my kids, etc. Although that seems to be changing with the advent of Madonna, but now it seems like only pop-stars are allowed to be hyper-sexual... and I think pop-star hyper sexuality is simply to appeal to a male audience to begin with and boost sales. I know most women don't care about Lady Gaga wearing a bacon dress with her ass hanging out, but I'm sure the male fans eat that shit up. I think I'm getting off topic, but the same logic applies to scantily clad female video game characters.
The Witcher is a very male-oriented series to begin with (although I've only played the first one), as are most action-adventure games. I mean, damn, the fighting genre is notorious for this kind of crap. Just look at Ivy or any other female character from the Soul Caliber series, then compare it to the male characters.
I can't tell you exactly why this is happening, or how come this trend hasn't changed (and it's about damn time), but it is interesting to say the least.