This is basically a reconsideration of my answer to a discussion on Physics SE about the question, if astronomy should be mentioned separately in their logo to make it clear to everybody, that astronomy is on-topic there.
To me personally it always seemed that astronomy is a subfield of physics, so a natural alternative approach to how things are currently implemented would be to put theoretical astronomy questions into the theoretical physics category, expereimental astronomy questions into experimental physics category, and so on.
However, looking at how astronomy is implemented in the academic landscape, it is not the case everywhere that astronomy is treated as a subfield of physics. Here in Germany for example, we have universities with institutes explicitely named "physics and astronomy", sometimes there exist distinct departments for physics and astronomy, or astronomy is listed as a part of the "mathematical and natural sciences" faculty, etc ...
So people from places in the world where astronomy is not treated as a subfield of physics generally might need some encouragement to ask astronomy questions here. Keeping astronomy as a separate top-level category in the Q&A section is a way to try to achieve this.