Two main drawbacks that i see for women in motorsport.
1. Physical abilities, women are genetically weaker than men, but with practice this wouldn't really matter.
2. Womens brains are wired differently than men. Take for instance the long held notion that women are rubbish drivers, well, alot are, and there was a psychiatric study recently (sorry forgot the link) that delved into this, basically it went along a few things, women give cars a name, men don't. Men see the car as an extension of their own bodies, whereas women see cars as a item they use.
But those two points only explain why there aren't a lot of women in motorsport, it's because it's harder for them to get into it, they aren't really interested.
Of course there's lots of exceptions, and that's great. And it also doesn't mean a woman can't be as good as any other man can be, it's just hinders them in the begining.
Personally i think women should be encouraged to get into motorsport, i always take my girlfriend along to my trackdays.