Apart from the fact that cost reduction is happening in general as time goes on and that NASA will be launching another, bigger, rocket shortly, it really does come down to it being "rockets that landed" as everything else has been done before and could already be done.
It's true the SLS will be more powerful. However, it will launch, at most, once per year. And at a cost of over a billion dollars per launch. Compared to the Falcon Heavy, which will be able to launch dozens of times a year at less than 1/10th the cost.
Full and rapid re-usability is something we've never seen before. Not even in the Space Shuttle. The next two or three years are going to be very interesting. And the upcoming BFR is going to be a further game changer. We're going to be able to launch payloads with near the mass of the entire ISS - in one go. And send 100 people to land on Mars in a single launch.
Until yesterday, everything in orbit cost nearly it's weight in gold. Now it will cost a fraction of that. In ten years time reaching Low Earth Orbit wont cost much more than a first class plane ticket. The importance of re-usability cannot be overstated.