Rebuild 904 or swap for 727?

B and m advertises their 904 kit (competetively priced) to make the. Tranny handle 500 hp
Built right it will handle more than that. Strictly from a drag racing perspective the 904 internals and design are very popular. Pro trans put 904 internals in a 727 case. Afaik ati and pro trans are developing a turbo 350 aftermarket Sfi case that will use 904 internals. Why is that ?