the difference between RD and RT

What's the difference between (RD) Route Distinguisher and (RT) Route Target?

--the purpose of the RD is to assure uniqueness of the VPN prefixes in the VPNv4 network and that the RT is used to determine which VRFs will import these prefixes.

Why can't RD do that?

--RD could have fulfilled both purposes. The problem is with more complex scenarios, where only using the RD would set a major restriction. Just think of extranet scenarios, where you want to share specific routes between two VPNs. This could not have been achieved using just the RD since you would have been forced to use the same RD in the two VPNs, which would have caused overlapping routes from these two VPNs to overlap in VPNv4. This defeats the purpose of MPLS VPN.

Using two different entities to play two different roles was a way to keep it more flexible.