North of Perth you're on a single-track line, and a 2+9 HST running on a single power car is required to have a clear, unchecked, run at the two big climbs on the HML. Perth is an unsurprising place to terminate it, given it'll presumably run empty back to Edinburgh (where the power car can be repaired, versus being in Inverness and then needing a clear run at the big climbs the next day, and another PPM failure for the southbound service and however many knock-on delays that has), and it provides onward travel on both ex-Edinburgh and ex-Glasgow HML services (though yes, coaches would've probably been good in addition!).