The Wankel (Mazda) rotary engine and the Rand Cam rotary engine are very different internally.
The first difference is fuel type. Wankel is gas and Rand Cam is diesel. Diesel fuel is much better as I detailed in a previous post.
The Wankel does not get much better fuel economy than a piston engine, it has sealing and wear problems, and it has emissions problems. Mazda may have worked out many of these with its latest engine, but it is still not that much better than a piston engine so why change. Developing any new engine costs many millions of dollars.
The Rand Cam rotary promises to be much better than a piston engine (and a Wankel engine) in important areas like fuel efficiency and emissions. Like the Wankel it will be smaller and lighter than the piston engine.