Good question. Maybe the Commentaries address a reason for that? I don't have access to all of the commentaries yet.
One possibility I have heard is that the Bodhisattas are all born from their mother's side, suggesting a Caesarian section was performed. In those days, medical care was no where near the quality of today and many women died shortly thereafter.
Apparently there have been C-sections performed before modern hospitals: http://en.wikipedia.org/wiki/Caesarean_section#History