Despite having negligible photorespiration and photosynthetic advantages, what are the factors or reasons that C4 minor millets failed to surpass the yield of C3 rice ?
Your question assumes rice has higher yields. It does not! In areas where minor millets are grown (central Asia, central Africa, and North America's central Plains), water is lacking for rice and rice has too long a maturity. C4 minor millets do outyield rice under semi-arid and arid conditions. That is why no rice is grown in Mongolia for example, but minor millets (proso and foxtail) have been a staple there for thousands of years. See Preprint Ancient Proso Millet and the Twentieth Century Survival of Mongolia