Stability, for one.
The React docs on Context say:
If you want your application to be stable, don’t use context.
It is an experimental API and it is likely to break in future releases of React.
In fact, the new version of context was proposed in December of 2017 and is on its way.
Presumably, react-redux will be automatically updated for you to use this new API. If you implemented your own context/proptypes passing of the redux store, you'd have to manually change your code to the new syntax.