Purely speculative/anecdotal, but: Atari 2600 games are just not that fun. Of course, for a while it was the best gameplay experience one could have, but when the next generation of consoles came, it was flat out outcompeted.

I (and quite a few people) still play NES-era games, and not just because of nostalgic value. A lot of them are still a lot of fun. I have tried some Atari games, and with hindsight and the internet, I can target the best ones, or even recent homebrews that were made with current knowledge about gameplay mechanics, but at best they can get a few minutes of my attention mainly due to curiosity. Gameplay is severely limited by the single-button controller and processing power, very tight memory constraints mean very limited content (at best a handful of different screens played in a loop), and graphics are horrible. My brain can "fill in the blank" with NES-generation or later pixel art, but not with Atari 2600-style big blobs of color.