I think while Atari cartridges can be found pretty easily, being another dozen or so years older than the Atari would likely mean there are significantly less of them floating around. Plus the Atari port would have to have the unlocking mechanism to that Genesis doesn't, to open the physical access to the cartridge board on a ton of cartridges. Given the expense of adding it would likely be a selling point to only a proverbial handful of us hardcore gamers, and therefore not worth the additional effort. There was the 'hackability' on the FB2 to add a port, and to be honest, it seems like very small number of people actually perused that. But of course, I'm just guessing here.