I think that the devs probably felt that some outfits wouldn't fit the opposite sex too well and hence the gender locking came to light.
And I also think there may be additional resources involved in trying to design the gear to make it fit onto the toon. Didn't Yoshi P mentioned before that because of the new Viera they had to look into the headgear design to make sure that it looked good on a Viera's head? Hence it could take up some time and resources for that, which would have been better spent on developing new ideas or improving certain matters on hand.