They had a nationwide 'electronic' casting call out a week or two ago (meaning, they were accepting pictures and sizing information) for girls from allover to submit for consideration to be Anna and Elsa face characters in the park. Disney has pretty tough standards for finding 'friends' of their princesses and then have to have them properly trained for the roles- it can take time. Plus they have to find girls willing to relocate to Florida, work the required hours and for the standard pay. It's not as easy as some would think it is to simply find a girl to 'put on a costume and meet kids.'
It also comes down to budget, as well. While Disney does make a very good deal of money, they still have to stick to an entertainment budget, and they simply can't pay to have a hundred girls to play princesses all the time. I think they are trying, though having them spread out to a few areas might help, I do think putting them on FP is a step in the right direction.