The size of React's ecosystem, bus factor and the availability of React Native all are nice but overstated in their importance relative to alternatives and obscure the many other considerations the WP community weighs up.
Further more, React's ecosystem size conveys marginal benefits given that use of React in core would have been constrained (it's not like core devs would be tapping into many additional React-based tools for core work).
React Native has not proven itself to be as stellar and promotes lock-in. If we come to rely on additional FB-engineered architecture to such a degree WP bets its future on the decisions made by a corporate giant with goals very dissimilar to WP and with decisions that may change down the line. This idea that FB has somehow has the closest thing to the holy grail and so we must all jump aboard the FB train is a very foolish and shortsighted one in my view. WP was wise not to adopt it.