Menstruation is considered to be "muci," or impure. For many brahminical families, temple worship and puja were their primary occupations. One cannot worship the Deity or perform rituals if one is unclean. The concern was that women would contaminate the offerings prepared.
On a different note, women were allowed to fully rest during menstruation and were excused from all household duties. This lead to quicker recoveries and happier women.