I think what killed it was their attempt to sell packs of cards separately for the game. I know I didn't like the idea. I have all 3 of the box sets, but not a single pack of the "collectible" cards.
They sold "booster" packs for $3-4 each. if I want to buy a bunch of boosters , hoping to get a good card, I'll play Magic. I don't.
Check EBAY, you can usually find a good deal on the new Gamma world boxes. Some decent art, interesting ideas, but I'll play Mutant future instead. But some of the ideas are useful in a GW , MF game.
The totally random character generation can be fun. If your players don't mind that kind of thing. If they want to choose what they play, then they will not.
The Basic GW set has 20 character types, Famine has another 20 types, Legion of Gold another 10. You can use them all, mix & match. There are a lot of types, Demons, Undead, Radioactive, Alien, Vampire, Seismic, Divine, A.I., Plant, Pyromaniacs, etc,etc. Its more of "crossed realities" than "post-apoc".