The biggest reason is that for most players, concrete is far easier to make early game than forged steel. Sure, you can scrap certain things and the quest rewards include enough for a whole 2.5 blocks but unless you have a very small base, you'll need to craft forged steel and that requires a crucible that you usually won't unlock for a while. Once unlocked and if you want to mine a lot of iron, you can easily do steel for your base. Until then, concrete is a great option.
And, as mentioned, steel blocks aren't new.