In part down to what is available, but vehicles presented for test have to fulfill certain requirements. They must have nearside and offside mirrors for the examiner. The examiner's seat must be belt fitted. They must also have a tachograph fitted and be equipped with functioning ABS system, be capable of traveling at 50mph and meet certain dimensions (10m minimum length and at least 2.4m wide).
Those requirements only relate to vehicles presented for test, trainers might be used for other purposes, including route learning, customer service training, etc so not all trainers in a fleet need meet the above requirements. The regs do explain why coaches were often employed though, until recently older coaches were more likely to meet the requirements than older service buses which would have required modification.
I seem to recall Western National once added extended bumpers to their Mercedes StarRiders to meet minimum length requirements.