Because if we are going to Mars, we need to build the ship in lunar orbit. Here's the thing that everyone seems to ignore about a Mars mission: once you are outside the Earth's magnetosphere, there is a high level of nasty high energy radiation. It's kind of like living inside a microwave oven. The only fix is to use inches of steel or feet of concrete for shielding. Now multiply that by the huge size of the ship required to hold food, oxygen, and other supplies for a two year mission to Mars and back.
Trying to get that much mass into orbit from Earth is practically impossible. If we make concrete from lunar rock, on the other hand, it is easy to boost into lunar orbit.
Two year mission is too long IMO. While I would love to see it and thought I would, I no longer think I will in my lifetime without some kind of breakthrough. The distances, time and radiation environment will prove to be too much of a barrier unless that happens (again IMO). I'd love to be wrong
