SNK has always struck me as a schizophrenic developer and publisher -- they had a number of solid arcade hits in the 8-bit era, many of which made their way on to the NES while they were a Nintendo licensee and also found their way onto 8- and 16-bit platforms of the time. There was even a version of Ikari Warriors released for the Atari 2600.
But after SNK brought out their Neo-Geo coin-op/high-end home platform in 1990, they seemed to enter a new era with a new philosophy. They began sublicensing their newest hits to other publishers for home console ports that were often sub-par (in one case, a character's "Big Tornado!" battle cry sounded like "Baked Potato!" on the Sega Genesis). Worse, they almost completely ignored their own heritage. The Neo-Geo had many fine games, of course, but we never saw true sequels to Ikari Warriors or Athena.
Maybe the move from a vertically-oriented screen to a standard TV setup was a factor in leaving Ikari Warriors, Victory Road and Ikari III: The Rescue as the last of their breed, and the change to standard joystick/button controls without an aiming spinner may have been a factor as well. But Athena was always a 4:3 game that would have worked perfectly well in a sequel on the Neo-Geo. At least she and the Ikari characters appeared in a few of the flood of fighting games the platform hosted, so they weren't completely forgotten. This was an unfortunate trend of the time -- Karnov and the Double Dragon characters also appeared in generic fighting games on the Neo-Geo and have not been seen since.
I was freshly disappointed when SNK Playmore released SNK Arcade Classics: Volume I recently, because while it does include 16 high-quality titles, they are all from the Neo-Geo generation. One emulator fits all, I suppose, but it implies that the current owners of these properties see no value at all in the pre-Neo-Geo titles. And that irks me.