Most cargo ships have a single main engine with plenty of backup-less failure points. They are sort of engineered so these failures can't happen suddenly but you can help yourself to a bunch of videos on how substandard fuel and parts shortages cause week-long poweroffs in a middle of the ocean.

System designers and regulators are aware that the main engine is a single point of failure, but they generally consider loss of main engine power to not be an immediate emergency. There are redundant systems to retain electrical and hydraulic power, and losing motive power isn't generally an instant emergency. Power and steering together is an emergency, yes, and steering is degraded without power, but had they still been able to use the rudder they wouldn't have hit the bridge.