Because NES Mega Man boxart has a history of being utterly retarded, honorable mentions going to Mega Man 1's
"ice cream man from the future visits Funky Land and stands funny and has a giant head tumor". Runner up goes to
that European boxart where Mega Man is silver or some shit and looks like something out of Lost in Space.
Also,
MM2 (US) boxart had Mega Man with a handgun as well, though he didn't have a second arm canon. I guess Capcom wanted it to be
really campy, hence the eyepatch on Wily.