[transient]

Well, I'm by no means an expert in this area, so feel free to correct me, but here's my understanding.

Steel is ultimately stronger than aluminum, but it also tends to be more flexible at the same time. Therefore something like a strut tower, which doesn't take a huge amount of stress, but needs to be very stiff, is much better suited to aluminum than steel. Aluminum also has the benefit of being resistant, if not impenitrable, to rust. It's much lighter weight as well.

One must not forget, however, that the engineering of a part has as much to do with the structural integrity of something as the material that it's made from.