Why doesn't Sprint do that on PCS? If they shared the transmit path there, they could do 4x4 without a second antenna…
The RRH has a limit to the number of carriers that can share a transmit path. For example, the Ericsson RRUS 35 supports a maximum of two LTE and six CDMA carriers simultaneously, but no more than 4 carriers can share any single transmit path. So to run the maximum number of carriers, you'll have two LTE and two CDMA carriers on one set of paths and four CDMA carriers on the other set.
There's actually a site in Central, Lousiana that does run 4x4 MIMO with a single RRH and antenna, but it's only running one LTE carrier and presumably no more than 3 CDMA carriers.
EDIT: I just noticed this was in the Samsung thread, so perhaps someone who is more familiar with that equipment could offer more details about its specific limitations.