Not speaking for any company/lens designer, but I think it's a lot less practical now than it was in the manual focus days when one hand on one ring controlled both zoom and focus. The push/pull zoom was less precise than a rotating ring (it took some small force to move from any rest position, so overshooting was easy for critical composition), but managing everything from a single ring made life simpler overall.

But to extend the question a bit: why did push/pull focus fall from grace? After all, for a unit-focus lens (a lens where all of the elements/groups are in a fixed relationship with one another, and the whole assembly is moved toward or away from the film/sensor as a unit), that's all you need (and you can, if you wish, use push/pull focus alone on most rail-type view cameras). Two things make it impractical -- a lack of precision on the one hand, and the fact that lenses on small-format cameras don't tend to be unit-focus these days (internal focus, which is actually a sort of zoom, and corrective elements that float in relation to the rest are closer to the norm these days).

With focus largely outsourced to the camera, the more precise two-ring arrangement is a lot less annoying than it used to be (like on my old Minolta 35-70mm f/3.5MD). With focus control being mostly a matter of a button-push with the right hand, the left hand is free to concentrate on the zoom. With a ring, the only real annoyance is backlash -- there's usually a small but detectable "dead period" -- just the tiniest fraction of a degree -- when you reverse the direction you're rotating the ring. With a push/pull, on the other hand, there's usually some stuttering as you overcome the friction (the "tension ring"), so making fine adjustments after you've done the gross positioning is sometimes difficult and frustrating. You might think that really, really small adjustments aren't nearly as critical as a lot of people think they are (and I'd be inclined to agree with you), but as long as photographers are convinced that they have no pixels to waste at all, precision is going to win out over speed as long as the speed penalty isn't outrageous.