Well this is a weird one. Talking to Ruffian Games creative director Billy Thomson at PAX East, 1UP got the skinny on why the Crackdown series has never featured playable female characters.
Thomson, who was previously the lead designer on the original Crackdown, says "the amount of memory that it was going to take for it to do all the actual animation, the texture sets and the models for the females -- we would've needed to massively reduce other sections of the game."
"It's actually the same reason again" for Crackdown 2, he explains. "We had to save memory all over the place because we've got so much more content in this game than we had in the first game. We don't stream." More in-game content or female leads -- seems like an odd ultimatum to me.
Crackdown 2 producer James Cope -- probably hoping to reassure us that Ruffian doesn't hate women -- explained how there would be "an important protagonist that is a female character, and it will become evident in the game that she is going to be very important for Crackdown as a franchise."