There are several reasons for that. For one thing, Irem couldn’t afford to market or distribute Metal Storm heavily. And even though the game landed a Nintendo Power cover, it didn’t catch on among a gaming press that was already inundated with NES releases.
I doubt that the M-308 Gunner itself had anything to do with Metal Storm’s low profile, although it lacks something found on most robots of its day. Everything about the M-308 Gunner makes sense until you notice that it has no head, and that big orange shape on its chest doesn't immediately register as a cockpit. Perhaps American kids just weren’t ready for a mecha without a face. Someone involved with Metal Storm’s print ad thought the exact same thing and decided that this robot needed a head. Any head would do.
So the advertised version of the M-308 Gunner borrowed a head off one of Japan’s most recognizable anime robots: the Zaku from Mobile Suit Gundam. As low-level enemy mecha, Zakus are destroyed by the truckload in various Gundam series, and by the end of the 1980s, they were almost as recognizable in Japan as the iconic white Gundam robots themselves. In America, however, no one would spot a Zaku’s head (or a Gundam-like shield) outside of a few devoted anime nerds who happened to page through a GamePro issue. Nobody at Bandai, which had canceled a Zeta Gundam NES game back in 1988, noticed this copyright violation either. They were too busy prepping Doozybots for that fall's TV schedules.
Metal Storm also might’ve stolen its name from a movie about The Destruction of Jared-Syn, but that’s another story.