Large community, so lots of resources available online
Conceptually simple, clean abstractions
Matured dev tools
functional programming oriented, which offers cool benefits like time travelling, trivial action testing and such
Actions and state changes are very traceable
Rigid paradigm to work in (which is generally speaking a good thing)
MobX
Very efficient out of the box (applies side-ways-loading without needing selectors etc)
Very suitable for a state tree that has lot of inter data relationships; the data doesn't need to be normalized to a tree
More OO oriented; you can use classes, instance methods etc..; less new concepts to learn
UI is always kept up to date
no flux like action dispatching required (although you could still do that). On large apps this saves tremendous amounts of boilerplate
actions are really straightforward; you don't have to return new data structures etc. You can just alter objects and the changes are picked up automatically