Why do trolleybuses power cables use DC and not AC?
Trolleys with overhead wires often disconnect momentarily and the high inductance creates an arc of the same current that allows some continuity but with some drop of voltage or power.  Whereas AC might cutout in &lt; 10ms.
But the real reason is DC traction motors have greater efficient torque needed to start.
You might see some DC motor Torque capability curve. These match the load vs speed requirements better than AC motors of accelerating a trolley car with constant current is applied by a control method until speed is reached.
Also another advantage in the Netherlands are the DC trolleys that have batteries to extend routes.   An 18-meter bus with IMC500 technology only has to travel 20 percent of its route beneath overhead lines.