Probably because they don't care. They created React to solve a problem they had, they open sourced it 'cause why not, some people adopted it, some people didn't...Facebook doesn't care either way. They're not an open source company, and they don't really care how many startups use React, or how many Github stars they+ have. They don't rely on external users for testing, or external devs for contributions.

It's not so much they're "digging in" in the face of criticism, as they simply haven't noticed the criticism, because it has no relevance to Facebook at the level where corporate policy is set. Or so I believe. :)