isn't it just aluminum plated?
Aluminum has its advantages and disadvantages over steel...
first off, it absorbs heat more effectively than steel
second, it is lighter and just as strong as steel
but, it doesn't absorb as much sound as steel does, so alu cases tend to be louder
and finally...aluminum is very costly to make, it takes about 4 tons of boride (<--not a chemist) to make 1 ton of aluminum? and it must be heated at a higher temp than steel