I believe it's because of the serrations, which are typically only done on one side of most knives. My guess is the same grinder that cuts the serations also cuts the plain edge, so it's easier and faster to just do everything on one side. Especially since that blade style has become the new official swiss army knife (which means they'll have to crank them out by the truck load).

Personally, I just take the serrations off and convex the edge. That's why I like the models that also come with a seperate serrated blade, I'm not really a fan of combo edges.