Tangentg

Just tell them "Don't knock it till you try it."

It's probably a graphics issue. People who grew up with the NES often scoff at the Atari 2600 because the graphics look primitive compared to their favorite retro system, but you don't need great graphics for a game to be fun. Try starting them in with Pitfall II, H.E.R.O., and Kung-Fu Master.

Also keep in mind that the upside of the NES generation mostly disregarding the Atari 2600 is that the prices for games on the system are still fairly cheap. If the 2600 was as popular as the NES then we'd probably be paying $30 for common games like Adventure or Centipede.