Not surprised. I would expect the Lonestar to sell more than the Prostar. There are few true truck model builders, at least in the US. In my estimation, the Lonestar sold more because in many car model builder's minds it is looks like a 1930's whatever car that can be modified into some weird hot rod or used as a back drop to some race car hauler. Basically used with some type of race car.
The Prostar is not a chromed-up tractor, like your typical Pete or KW. When you see them on the roads, they are generally fleet prime movers. Not very glamors.
I hope Moebius continues to offer new big rig truck subjects in the future, but...