When altar service starts, the kids are at that age where the boys think girls are "yucky" and the girls think the boys are "slow" because girls are developmentally a little ahead at that age. But boys, having been taught that a gentleman does not hit a female, have no real way to respond to the disdain with which they are treated by the girls. So they avoid them.
When you force the kids to work together, the boys just passively drop out because they don't want to deal with all the conflict and drama. So you have nobody left but the girls, plus any boys whose parents MAKE them participate.
I've experienced this in both a "high church" Episcopal parish and our current Catholic parish. The problem is pretty stark, and these two parishes solved it in different ways. The ECs had all-male and all-female "teams", which channeled the animosity into team rivalry - not ideal for service on the altar, but better than the constant pick-pick-pick. The Catholics solved it by making the altar server program ultra-military in nature, with drills, rank, promotions, insignia, etc. That winnowed out most of the "girly-girls" and made things more attractive for the boys. I would say that the program is about 50/50.
But aside from practical considerations, and aside from the obvious point that a female Catholic altar server can never become a priest, there's another point: the Catholic Church is becoming very feminized, because when you have ladies running things they tend to just sail in and take over, and the men are happy to sit back and let them do it.
I'm not the only one observing this: