React does the diffing on the output (which is a known serializable format, DOM attributes). This means that the source data can be of any format. It can be immutable data structures and state inside of closures.

The Angular model doesn't preserve referential transparency and therefore is inherently mutable. You mutate the existing model to track changes. What if your data source is immutable data or a new data structure every time (such as a JSON response)?

Dirty checking and Object.observe does not work on closure scope state.

These two things are very limiting to functional patterns obviously.

Additionally, when your model complexity grows, it becomes increasingly expensive to do dirty tracking. However, if you only do diffing on the visual tree, like React, then it doesn't grow as much since the amount of data you're able to show on the screen at any given point is limited by UIs. Pete's link above covers more of the pref benefits.