Dave,
Actually, three of the original five were dropped. Two concepts from Sikorsky and one from Boeing.
The three concepts that were dropped probably had the lowest technical risk. But apparently, they also did not give the performance leap that the program was looking for.
As for the VDTR, Sikorsky spent a great deal of time and money looking at that. And in the end they apparently came to the conclusion that the rotor's mechanical system would be too complex and unreliable. So I don't know why the JHL program would go back down that path. A variable speed drivetrain is much easier to do than a variable diameter rotor.
Regards,
riff_raff