1. Somewhere in FB's patent portfolio is a patent that is broad and vague enough to cover some part of React, so the patent grant/revoke clause has meaning and value. Changing the license would mean that they were giving up something that they value, and they don't want to do that. That might not be true for RocksDB, in which changing the license is relatively low cost.

2. React is more widely used and more easy to detect (public facing websites) so the patent clause offers protection against a wider range of potential threats. Lots of sites use React, or react-like libraries.

Since the purpose of the patent clause is as a protection againt offensive patent suits, they seem to think (probably with good reason) that React is a worthwhile shield that they don't want to give up. For RocksDB they feel that balance is different.