Partly, because the minute you have a shared widget across users 50%+ of your connected users are going to get an update when anything changes. So the overhead of tracking who should update when you are under high load is just that, overhead.

Being able to make those updates corse grain and homogeneous makes them easy to throttle so changes are effectively batched and you can easily set a max rate at which you push changes.

Same with diffing, the minute you need to update most of the page the work of diffing is pure overhead.

So in my mind a simpler corse grain system will actually perform better under heavy load in that worst case scenario somewhat counter intuitively. At least that's my current reasoning.