Generally diesel engines are more reliable than petrol engines and easier to maintain ......
especially when one sticks to Euro 5 diesel that's available nowadays.
There is a reason why buses and lorry trucks/trailers use 'solely' diesel engines.

Agreed with low end torque.
But modern day turbo petrol engines have low end torque too, hence diesel's advantage on low end torque has been narrowed, if not eliminated with modern turbo petrol engine technology.
Yet, one doesn't find turbo petrol engines that could deliver far higher power density than diesels in a modern day say, trailers or heavy trucks let alone buses.

Fair enough, but buses/lorry/trailers need the power on roads/highways like changkat jering, KLIA towards Nilai, Karak etc.
Ooh, offhighway trucks in construction sites would love more power, if only there is ease of maintenance on turbo petrol engines, which aren't available to construction equipments anyway.