Well in the States at least the SNES did so well at least partially because of the success of the NES. I mean, up until fairly recently people still said "playing Nintendo" when they meant playing video games. I'm sure many old folks still do. So the SNES was a guaranteed success either way, especially when so many of their franchises were household names, and they dominated the third party market though less than savory business practices in the 8-bit era. It's amazing SEGA could break through as well as they did at the time with the Genesis.
I grew up playing the Sega Master System and I think I knew one other kid who had one. The Genesis was a no-brainer for me, and I just enjoyed the types of games which were released for SEGA systems, and still do. I had seen and read about the TG-16 as a kid but getting a new piece of hardware was a pretty rare event for me, so I went with what I knew, and I think that's what a lot of people did with the SNES. Plus a lot of great games from third parties such as Konami, Capcom, Squaresoft, Hudson, etc., came out for the system along with some first party gems. So to me I think the success of the SNES has very little to do with the hardware but with the marketing, demographic appeal, and a decent library of games well-supported by third parties. Nintendo has been successful at grabbing the casual and mainstream market with the DS and Wii as well, and it works for them, but for me I don't really see the company are the pinnacle of gaming as so many people actually do. But I think I'm preaching to the choir here! Sorry for the wall-o'-text!