It isn't straigthforward to port a total-conversion mod from R:TW to M2:TW. Some things can be transferred fairly easily, but others require a lot of work, and to actually remake the mod is something else entirely. That's one of the reasons why the team decided to start afresh. (With hindsight, of course, it is taking a lot longer than planned.)

And M2:TW is simply a development of the R:TW engine. The newer TW games use different engines. Even if CA decided not to reuse the S2:TW engine, the chance that they go back to the R:TW/M2:TW one is nil. Hence we cannot expect any backward compatibility between R:TW and R2:TW.