The NeoGeo was always much more of a niche system than the NES. The sales method (collections of games on memory cards) and initial piracy holes (you could load up all the ROMs onto an SD card and never have to buy any collections.
Also, the hardware was crap. Even though the games were made for 4:3 monitors, the system shipped with a 16:9 phone screen (likely because 4:3 screens are harder to source cheaply nowadays).
the screen was not appropriate but the hardware was not crap. the controls used very high quality click-joysticks, that you find nowhere on portable consoles.
Nintendo has probably sold hundreds of times as many home systems as SNK (impossible to know exactly as SNK doesn't seem to have released numbers), which I'd predict is a fair indicator of how comparatively successful Nintendo's system will be.
