This is just a guess, but I assume it is a weight/design issue.
Only one liner is needed for a lock. The Wegner has the unusually long, steel back spacer, which weighs quite a bit. The Wegner as is, is a reasonably hefty knife. A second liner might add a bit more rigidity, but at the price of additional weight, probably making the knife too heavy for it's size.
Of course, the back spacer could have been shortened, or done away with, but that would require adding a stop pin. As it is, the Wegner's stop is on the inside of the backspacer, much like a lockback. I don't know of any other liner locks that utilize the backspacer, rather than a stop pin, like the Wegner. Frankly, I like it.
There are some other liner locks that only have one liner. That isn't unique. The first that comes to mind is the CRKT M16. It has aluminum handles, so no rigidity needs for a second liner.