Travel to the heart of the Peak District in Train Simulator Classic, the Hope Valley Line is out now on Steam from Just Trains. Read More : Hope Valley Line - Out Now! (dovetailgames.com)
Most likely because the class 166 is included in the European loco and assets pack which is bundled in with some routes. The class 101 is a separate purchase so not everyone has access to it.
It's all down to the developer Just Trains as to which stock they include for default scenarios. As I already own the route from the Just Trains website, I couldn't comment more as that one includes default scenarios as well as other stock from AP.
Slight bump, just bought this in the sale with my first run a quick drive from Manchester to Sheffield with the 7MT via Marple. Very nice route, almost TSW like in appearance and detail. Shame about the use of default stock particularly the 166 which will stick out like a sore thumb, but nothing that can’t be amended with a bit of cloning and a few minutes in the editor.
For Steam, JT always uses default stock so everyone can play it. If you buy from JT directly, you'll get scenarios using advanced stock (you have to buy separately). Their Wessex line even comes with two versions of each scenario.