Truck manufacturers measure fuel efficiencies to the tenths and compete on that. At highway speeds, ICE engines win over a pure electric system, so throwing in another conversion step hurts. For city routes, pure electric is better than diesel-electric.

So in general, the disadvantages of a diesel-electric system outweigh any advantages when you take a look at how trucks operate.

New trucks have "automatic" transmissions (not torque converters- they're more like automatically shifting standard transmissions), so no manual shifting required.

Somewhat related, but refrigerated trailers ("refers") often use a diesel-electric system because it's easy to plug in to shore power and cut the diesel engine.