I think a large part of the problem was the cost of discoverability. On modern PC and mobile, with the availability of free games, it costs nothing but time to wade through piles of bad games. Even on the C64, piracy was prevalent enough that it was easy to obtain and try a large number of games.
Atari 2600 games required purchasing cartridges and therefore an outlay of money. Without the internet, it was difficult to get reviews or feedback on the quality of a game without knowing someone that actually purchased it. Added to this, most of the games sold at full price, regardless of the quality. It doesn't take the purchase of too many full price games that were really bad before one gets very hesitant to lay out cash for a new game.
Atari 2600 games required purchasing cartridges and therefore an outlay of money. Without the internet, it was difficult to get reviews or feedback on the quality of a game without knowing someone that actually purchased it. Added to this, most of the games sold at full price, regardless of the quality. It doesn't take the purchase of too many full price games that were really bad before one gets very hesitant to lay out cash for a new game.