Women are usually discouraged from doing science and maths based subjects, not because they're worse at it but because it's seen as a 'male' subject.
If you look at the ordinary olympiads, even though they're open to both genders, you'd probably find that it is mainly guys that are taking part because girls have been discouraged from doing it.
Having a female only maths competition is probably an attempt to correct that and to encourage more girls to take part and take an interest in maths in a less intimidating environment.