Presumably they share most mechanicals with the solo Citaros that, presumably, have better fuel economy.
As these were originally specified for use in London (?) would the gearing be different (particularly due to the extra weight and drag of the 'trailer').
Comparison should be made with double-deckers (in terms of carrying capacity).
From Wiki:-
The main benefits of an articulated bus over the double-decker bus are rapid simultaneous boarding and disembarkation through more and larger doors, much larger passenger capacity (120+ versus 80−90), increased stability arising from a lower centre of gravity, smaller frontal area gives less air resistance than double decker buses thus better fuel efficiency, often a smaller turning radius, higher maximum service speed, and improved accessibility for people with disabilities and the elderly.
One disadvantage that is sometimes experienced with some articulated models is the insufficiency of the effective motive power available to them. It is common for articulated buses to use the same engine type as non-articulated buses.
The extra weight of the bendy bus gives rise to a slower speed and acceleration.
The cost and weight of the strengthened deck framing and staircase of a double-decker was lower than the cost and weight of the additional axle(s) and coupling mechanism of an articulated bus.