“Master“ implies that the contents is authoritative somehow, as in “master copy” (meaning 13 in https://en.wiktionary.org/wiki/master#Noun). “Main” doesn’t have that connotation.

When one is willing to discard that connotation, then, if anything, “default” would be a more accurate name, because the fact that it is selected by default in certain situations is really the only technical difference compared to other branches.