the RX diversity at the BTS gives a gain to the UL signal, right ? That’s a good thing because the UL signal is slightly unfavored : MS power is weaker than the BTS power.
So the RX diversity at BTS provides a balance between UL and DL path, that is otherwise not done.
Diversity in the other direction is not needed, to maximize the cell capacity. It would only favor DL coverage compared to the UL coverage.
On top of that, it *was* hard to implement two antennas on a mobile station, in the nineties. I might be done in the future though (i think 3GPP has written about it).