Thanks for the Q
Here are the reasons:
1.In TDD,The UL or DL signals must arrive at the RBS or UE at a specific time to avoid interference. So it is hard either for the RBS or UE to setup more radio links and be synchronized.
2.Code Limitation,only up to SF 16 can be used in TDD.
However,3GPP is in process of implementing Soft HO in TDD.Maybe we’ll get it soon