Because IMO steel is an outdated and cruddy material. U6 all the way baby.
Steel frames ive heard, even the light ones, are still a lot heavier than an alu frame. If an alu frame was the same weight as a steel one, then it would be stupidly strong.
The fact steel is shorter and therefore lighter doesnt help your arguement, it helps mine, bceause if they are 9cm shorter and still heavier, then imagine a 1100 wb steel frame, it would be like riding a concrete block.
I wouldnt even buy a second hand steel frame, bceause they are still really expensive, you could probably get like two or three 2 year old alu frames for the price of one steel one.
If there are any real benefits of steel frames, then sure, go for it.