A buddy of mine was looking at 5.56 AKs the other day, and he noticed that only one of the in-stock options was threaded for 1/2x28. Is there a reason for this? There aren't nearly as many options for suppressor mounts for the metric thread most of the 5.56mm AKs seem to use. Is there some quirk of the AK design or parts supply that makes the metric threading more desirable?

I think it is mostly to appease the Ak purist. I would much prefer them to be concentric 1/2x28 or 5/8x24.

Yep, this.

Most models based on the -74 and -100 series guns have the standard threading as would be on the "comblock" made versions, usually m24x1.5, to maintain compatibility with other common muzzle devices and so on.  These threads are actually part of the front sight, which is pressed and pinned on the barrel.

There's a few of those models that have the 1/2"-28 thread, as well as the "AK 556" model which is a PSA original design.