I don't know why certain old school games from the early NES era don't have credits. Think I heard once there was a kind of stigma involved with game development in Japan during those days, like some people wouldn't even admit to their families what they do. Dunno if that's some myth or has some truth to it.
It's sorta like with Mega Man. For the longest time nobody really publicly knew who the creator/director of the first two games was. We had a guy in the credits named "A. K" who was the "planner" of the first two, and Inafune mentioned him on numerous occasions, calling him his mentor, without ever giving a name. But apparently that dude's name is Akira Kitamura, and I don't think that was made public until a few years ago. (In addition to directing the games, he was also a co-designer of Mega Man. I think I remember reading he did the original sprite, probably based on someone else's design [there are a few names for "character designer" in the first game], and then Inafune put ink to paper to actually draw the Blue Bomber.)