What was the rationale behind making size of pointer and usize the same?

This is something that's starting to show cracks, although so far only on a research architecture. But it is something, I think, we can see come down the pipeline to regular usage in years to come.

I'm interested to learn what was the rationale behind requiring the sizes of a pointer and usize the same, and along those for the lack of an uintptr type. Was there even one, beyond "it is so on any mainstream modern processor"?