It's also steel, which is probably better in an electric bike where you don't care about weight as much but you do care about durability.

Good steel frames are not necessarily significantly heavier than aluminum frames but they ride much much better (have more flex). They also can be repaired if required while aluminum is probably toast after anything that damages the frame. In anything that is not a race or cargo bike I’d always prefer a steel frame.

Steel frames are only easy to repair if they're lugged, i.e. the tubes are brazed into lug sockets. This was once a common method of hand building frames, but it's now rare and expensive. See https://en.wikipedia.org/wiki/Lugged_steel_frame_constructio...

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)

Why wouldn't you want a steel frame in a cargo bike? That seems like an ideal application for it? Or is it about weight?

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.