Fair has less to do with it than tradition. The best guards that PR ever put out were all-female (with the possible exception of 08). Regiment is one of the few premier groups that is actually trying to maintain SOME of their traditional roots, and it works for them.
The ladies on the guard are not all "110lbs or less" either, by the way. Truly, some are. But the average guard member now is about 5'6 and likely closer to 140ish lbs or so.