It's not because we question the abilities of women to be effective leaders, it's because the long shadow of male dominated leadership still dominates how we think about leaders.
We continue to talk about and lust after leaders who are "strong" and this is often textured in male features. This sets up the characature of women as softer, which means not strong in the masculine sense. Male dominated cultures question the idea of "strong women leaders" for any numbers of reasons.
We need new conversations about the relevance of gender in consideration of leadership and indeed about the whole notion of what constitutes the qualities and habits of good leaders.