Getting from streetcar to trolley bus was insofar attractive because it was known technology, and infrastructure (substations, suspension for wires etc.) was already in place.

Trolley buses do have inherently a longer life span, but are accordingly more expensive. I believe to remember reading that in some cases (could have been Boston), they took the electrical equipment from old trolley buses and reused it in new ones.

It has already been mentioned that trolley buses have much more power available, because they are not limited to the output of the diesel engine. And nowadays, maybe half of the power output of the diesel engine does not go into traction, but all other auxiliary components (mainly air condition). A trolley bus does have the same requirements for auxiliar power, but can simply draw it from the wire, in addition to the traction energy.

Modern trolleybuses are primarily single or double articulated (that's where the internal combustion engines start to get weak). In fact, they are considered as in-between step between a bus route and a light rail route. Part of the infrastructure (substations, masts, etc.) can be reused, and it can be a smaller financial load to the city.