The awl can only be included as the back tool on the opener layer. The opener backspring is the same regardless of knife configuration, so the awl doesn't sit any lower on 3 layer+ knives than it does on the 2 layer Tinker/Spartan. So why the nail nick cut out for those? I don't know for sure but here's my theory:
The nail nick cutout for the awl doesn't appear until about 1974. The small awl appeared in 1951, so that's 20+ years of not having it. Plus no 84mm knife has ever had it that I know of. So why only 91mm knives 1974+? 1974 is also the time that the fine screwdriver started to appear as a back tool (vs the Technician style pen blade replacement). The original fine screwdriver sits really low in the backspring and can't be opened unless there's a layer cutout for it. On models with the fine screwdriver, it's always on the layer next to the main blade layer and there always a cut out for the nail nick. So when they introduce the fine screwdriver for the Champion, Craftsman, Ranger, etc, they also have to start making liners with the nail nick cutout. Since the have these liners now, it's easy enough to include them in construction of 2 layer knives as a "nice-to-have" to make awl access easier. Not something that was worth redesigning the liner for, but since they did for the fine screwdriver, why not use it.
Totally unconfirmed theory but one I think makes some logical sense.