The reason that boys are celebrated earlier than girls is that along with the implication that boys generally become successors in families, there was the fact that the survival rate of boys was less than girls until modern breakthroughs in medical technologies had developed.
男児が女児よりも早く祝うのは後継者としての意味合いもあるが、医療技術が発達する現代までは女児よりも男児の生存率が低かったためである。