Cargo bikes require significantly thicker frames to sustain the load, so the weight difference is much more pronounced. They’re also quite heavy to begin with, so shaving an few kilos off is most often worth it. There are some bikes that actually have mixed steel/aluminum frames, Douce bikes for example use steel for the back part of the frame where the rider sits and aluminum for the front cargo section.

Compared to aluminum frames, all steel frames are more easily repaired. They can be welded without special equipment, put back in shape when slightly bent etc. They also fail less catastrophic - if an aluminum frame or fork receives even a slight bend in a dooring accident it needs replacing since it may just break clean though at the next inconvenient occasion (like hitting a slight bump downhill with high speed)