I have always thought it was something to do with "philosophy" sounding like something you would learn in school, while "sorcery" is very obviously magical. As a kid growing up in the US I had never heard of a "philosopher's stone," and honestly if the book were named that when I was in grade school I may have been more skeptical about reading it.

Broader appeal? I was always under the impression that it was religious sensitivities behind the name change.