The loss of the elemental punches really did hurt Zam a lot. Whereas Gengar's better STAB moves and the +10 accuracy on hypnosis, it's hard not to choose Gengar. Then with the addition of Gengar going "Focus Sash, Counter, Explosion," it just makes him fun to use. :p
Zam did get a few things, yes, but he's still much to fragile to use, especially with Choice Scarf and new priority moves to make the classic psychic dull in comparison to his brothers and sisters.
Besides, if I need a psychic, I'd use Gardevoir, being slightly more sturdy on the special defense side and versatile with status and support.
It's just a shame that Haunter looks so much cooler than Gengar. :p