Because Stagecoach are stupid, and an awful bus company.
Look across Europe, from Austria, to Italy to Sweden, almost every city has MAN Lions City, because they are high quality and reliable buses.
UK buses, are actually quite bad when compared to the rest of Europe IMO
ADLs in particular are the reason why I hate bus travel in the UK, as one will almost certainly turn up, it's horribly uncomfortable and it slowly rattles itself around town, the suspension is rock hard so you really feel every bump in the road.
All in all a miserable experience, and one which I'll only do if I absolutely have to. In fact I'll be getting a car soon, which is a lot better than a bus!
The Lions City is a much more pleasant experience that I don't mind so much, the seats are comfortable, the suspension is nice and soft, so it's built a lot better, so it doesn't rattle so much, so you waft about in relative comfort.