(1) The introduction of altar girls began as an act of disobedience.
(2) Traditionally, the acolytate is the early training ground for priests. The introduction of girls, who cannot be priests, interferes with recruitment for the priestly ministry.
Christianity took over from religions which featured Goddesses, Priestesses, Ritual Prostitution, Dancing Girls,and Sacrificial Sheep,Goats,Bulls,Horses,and Humans.
Christianity marketed itself as the antidote to these practices.
Questions like this simply amaze me. Have you taken a look lately at what's happened to the Catholic Church since liberals and feminists have had their way? It stands in a shambles, reeling from all the errors of liberalism, feminism and worldliness. Every Pope for nearly 2,000 has "backed" what Pope JPII is trying to do now; which is to bring back sanity and the traditional form of worship to the Church. Why do you even ask such a question?
It is sound Biblically, if being an alter boy (or alter server) is supposed to be sort of a pre-cursor to the vocation of the priesthood. Women are not supposed to be in pastoral positions in the church body. Woman are allowed to minister to other women, but not to men. To me, having girl alter servers is the first step to accepting women priests. Incrementalism, as practiced by the Fabian socialists. To me, it's part of the whole "goddess" movement.
Perhaps they don't trust the priests. There is a history there...for those that haven't been keeping up with current events.