It feels awkward for spending so much R&D for a sensor technology and then abandon it.
Do you see it awkward to spend much R&D on film and then abandon it?
You know, Kodak built the first ever digital camera and then abandoned it. That was very awkward, far more awkward than what Nikon did, as we all know the digital camera later reappeared and displaced film! (Bell Labs built the first CCD sensor.)
Perhaps they thought the sensor technology might be at true revolution, and then stopped development after realizing it will never win CMOS. A similar thing happened in the automotive world: Mazda developed the Wankel engine (also known as rotary engine) which they used in their sports car RX7 and then practically abandoned it (it's not completely dead yet, though, they are still using it in some very rare applications). It's useful for all of us that every single pathway is thoroughly explored, even if most of them will never displace the state of the art.
Or perhaps they failed to understand exponential growth. CMOS sensors might be a bit trickier to manufacture than some other sensor types. But, exponential growth in yields1 will quickly make what was once impossible into something very feasible.
(1): I recall a story told in microfabrication course about the impossibility of an integrated circuit. The yield for a single transistor used to be let's say 90%. Now if you package 100 transistors into an integrated circuit, yield is 0.9100 = 0.00002656 or 0.002656%. Fortunately for us, yields have improved exponentially and thus current integrated circuits have billions of transistors.