Because that portion is digital. Digital is fairly immune to noise, but it can produce a lot of noise. The power distribution network can carry that noise to other components that it passes over, but because that section is all digital, it's ok, it's reasonably tolerant if it.
The other portions that are uncovered have more sensitive analog circuits. You need to be more careful of how you route power, to avoid noise, so you don't cover it all in a big network on the top metal layer, but instead it's routed more selectively.
The other portions that are uncovered have more sensitive analog circuits. You need to be more careful of how you route power, to avoid noise, so you don't cover it all in a big network on the top metal layer, but instead it's routed more selectively.