WRTD only concerns itself with event distribution. WRXI on the other hand aims to be a complete instrumentation framework. As such, WRTD is a part of WRXI, the part that handles distribution of events between the instruments. Anything not related to event distribution (e.g. configuration of an ADC acquisition, data retrieval) is not part of WRTD, but it belongs to WRXI.
I looked at the API of WRTD and it looks very similar to IVI/LXI. Why is that?
Since WRTD is part of WRXI, and the aim for WRXI is to be merged into IVI/LXI, WRTD was designed in the same spirit, to reduce the amount of modifications that will be needed in order to merge it with IVI/LXI.