Not merely that she was president of the association, but that she was the first woman to hold that post.
She is a poster child for women's equal rights. The WH is playing to the female wing of its support constituency (as well as the evangelical wing and party-faithful wing). Of course, there is some overlap in those constituency groups, and none of them are "all in" or "all out."
Plus, the year Ms. Miers was elected, the powers-that-be in the State Bar decided it was time for a woman president and chose two women as the candidates.