Ah, darn.
I hope they come to other carriers (I'd almost go so far to say that I expect it), it was worth the wait/premium price if you ask me.
As for 64gb, I suppose speculation is the best guess. I'd fault it to some of these:
-64gb hasn't proved to be popular perhaps? Not enough demand?
-Expandable memory is a wonderful snake-oil for the average customer
-The average customer doesn't need a 64gb phone...market just isn't big enough. You can see this with the limited 32gb supply in itself. Overseas a lot more people use their phone in place of a computer, so the larger 64gb size is much more applicable there than here (arguable, but logical).
-Cost vs. revenue issues (ties into the above I'd say).
I said it before it was announced and I'll say it again. The 32gb wasn't available at launch so those who couldn't wait would flock to what cost them the least to produce, the 16gb, and right at the two week return period the 32gb would be available. Surprise, that's what happened and left a lot of people unhappy.
While it's a smaller market, they're doing it with the Google Edition S4 too, though that's kinda a situation that is in a league of it's own as pricing and availability is such a different factor.
I guess ultimately is COULD happen but I surely wouldn't hold your breath. We got lucky to even see a 32gb in the states for the S4 given the difficulty to obtain a 32gb S3 here when it was in its prime.