Why does the name 'galaxy' apply generically to structures like M31, but the specific name 'Milky Way' is reserved for our home system?

Milky Way retained its ancient, descriptive name established by naked-eye observation before the true scale of the cosmos was known.

The difference in naming convention stems from the chronology of discovery and perspective. For millennia, the Milky Way was the only galactic structure humanity was aware of, leading to its descriptive, mythological name based on its visual appearance (spilled milk). When later discoveries, such as those concerning M31 (Andromeda Galaxy), confirmed that other, external 'island universes' existed, the term 'galaxy' became the necessary generic scientific classification for that entire class of massive objects. Since 'Milky Way' was already deeply embedded in culture and science, it remained the unique, specific label for our local system, analogous to keeping a unique house name even after discovering millions of other houses.