In 2014 the Orion spacecraft was launched on top of the Delta IV Heavy for the first time. Why do they build another rocket, the SLS, instead of launching the Orion on the Delta also with humans on board? It surely would be fewer costs to rebuild the launch pad to fit a Delta Heavy with the Orion instead of building the costly new rocket. What's the reason that they don't use the Delta Heavy for crewed flights to the Moon and beyond?
The following link provides answers why another rocket, the Falcon Heavy, is not used: Falcon Heavy question on SE
I think the proposal automatically came up since I didn't edit my question since then (I edited it before).