The "why" part seems critical to this.
I'm not a "gamer". I play video games, I enjoy them, I like to collect them to be able to play them. But since I sorta jumped the boat after the 16-bit era, I missed out on a lot of the development in the PS1/N64 era. The big leap into 3D and what I consider "modern" gaming.
I tend to pick up systems/games a few years down the road. They're much cheaper, and if I'm not chasing the latest thing, even something 2-3 years old seems new to me (I just don't know any better!). I've got an Xbox360 with a stack of games (usually from series I know I like -- Arkham series, Halo, Gears of War, Bioshock, Saints Row, etc.), but those are "sit down and invest time" games for when I want to spend an afternoon in a virtual world. Since I didn't make that transition to "gamer" back in the day, I'm much more of a "30 minutes here and there" player. Casual.
Which makes NES, SNES, and (naturally) Atari games appealing to me. I CAN commit lots of time to playing them, but they don't necessarily require it. I can pick up and play as much or as little as I want. As a non-hardcore, that appeals to me.
The nostalgia factor doesn't hurt, either. I'm almost 33, so an Atari was essentially the first computer of any kind I'd really ever used, the NES was a cultural phenomenon for kids when I was one, and the Super NES had some of my favorite games in the early '90s -- most of which were just distillations and refinements of principles from previous generations. (More detailed side scrolling beat-em-ups like Maximum Carnage -- for better or worse -- principally expanded on the gameplay of games like Double Dragon, Contra, et al., in ways I really liked at the time.)