Every sale has a limited number of seats available. This includes both the Church Choir presale, as well as any other presales (Radio, venue) and the public onsale.
I am not sure of the exact reason why the presale closes hours before the public onsale. However, every presale I have ever seen for any venue or show has been run that way. Maybe it has to do with getting an inventory before the public onsale starts so they know how many tickets are remaining. Doesn't make sense to me but I'm not in the business either so I'll check with a friend who is.