A related question: why not make touring frames of aluminum?

I was looking at Rocky Mountain's steel-framed Sherpa 30 tourer, and I noticed that, though it's obviously equipped differently, the frame geometry is extremely similar to that of my aluminum-framed commuter.

So given that so much else is equal, why the difference in frame materials? The [supposed] ride quality? The [supposed] weldable-in-rural-Africa factor?