The General Public hates add-on when it comes to game systems. They want simple and don't see the need to pay more to play games on system they already spent a lot of money on and already has a bunch of games available that doesn't need an add on.
From the Sega 32x to the NES Aladdin Deck Enhancer, they just don't succeed in the long run. And publishers want to target as big as an audience possible, so the go with the main system more times than not. Even the Sega CD, which was one of the most successful add-ons, failed to reach 10% of the user base of the Sega Genesis. And remember when Nintendo made the Expansion Pak for the N64? The basically gave them away with copies of DK64 and some systems to get users to adopt them. And even then, most N64 games did not take advantage of it.
The gamer geek in me really likes add-ons, but for practical economics it seems to always fail in gaming.