Previous incarnations of the Mercane/Arcane were considered Outsiders (innately extraplanar beings, like angels, devils, etc.) of one form or another, whereas the original 5e printing had them classified as Giants.I'm not sure about the Mercane. In fact, I'm not sure why they are Celestials now.
Revising them to be Celestials is presumably to restore their status as extraplanar beings, especially if they're doing something with Giants as a category in the upcoming Giant book that would have clashed with what Mercane traditionally are. It does, admittedly, imply a level of innate "goodness" that has never been a Mercane thing, but then we don't really have distinct categories for Outsiders beyond Celestial and Fiend in 5e.