Yeah, but an iron pick only does about twice as many blocks as a stone one, and iron is far, far less common than stone. I simply would not have had enough iron to dig out that whole area. I had to dig out roughly 145,000 blocks. An iron pick can mine about 250 blocks, so that's 580 iron picks or 1740 iron bars (over 27 stacks.) Digging out that whole area netted me about half that much iron. I figure it would've taken me about as much time to find enough iron as it did for me to dig it out with stone.
And doing it with diamond would've required me to cheat.