Through the eyes of grade-school boys, girls have cooties. (This is not the same as older boys and adult men not being able to interact with girls and women. It is a normal developmental stage.) If girls are serving, boys who want to avoid cooties will not serve. Pretty soon, it’s more than 90% girls, and boys are not interested in participating in a “girl thing.”
Complicating all this is the fact that girls often do the job better than the boys. So the girls get praised and thanked and held up as an example, and the boys get corrected and made to feel inferior. Yet another reason the baseball team or soccer league looks better to them than serving at the altar.
If altar service is restricted to boys, there are no worries about cooties, doing a girl’s job or being compared unfavorably to a girl. Boys are able to watch the priest at work and express an interest in doing the same thing. Being at the altar becomes a masculine thing, and I think we all agree that we want our priests to be appropriately manly.
So this was my long way of saying that I think it best that only boys serve at the altar. Girls can develop their religious vocations by doing activities more closely related to religious life - volunteering to tutor, collect and serve food to the poor, etc., instead of watching the priest work when they will never be able to follow in his vocational footsteps.