Both the uni-directional data flow and virtual dom were not exactly new ideas when React/Flow came out. There were other libraries/frameworks playing with similar concepts at the time. Half a dozen new ones appeared in 2014. React won the lion's share of developer adoption.

I personally heard that 'objectively better' line incessantly from 2015 onwards, with 'it has the largest community' always being a top highlight. Facebook's backing and the celebrities that developed in the JS community around the time certainly played a part in it.