Serious question: I remember when Mazda first came out with its Wankel and all the hype was about less moving parts, cheaper repairs and efficiency. So...why haven’t Wankels taken over as the primary ICE? To me it seems a huge failure. But I make that observation via casual observation based on the lack of its prominence vs the original hype.
There are many small Wankel designs in use in various applications. So it's not a forgotten technology.
There are a lot of factors in the selection of engineering technology in the multi-billion dollar automotive industry. The engine is one part of a system with many components. The whole system is the selection, not the engine. Decision makers tend to favor established technologies unless new ones are superior enough to be worth the risk of the negatives. If you look at the history of the Wankel you'll find it coming up short in the face of key decision points, like the fuel crisis of the 70's, emissions control regimes. A lot of these things were overcome, but not in time to change industry direction.