Mars is at the bottom of a fairly deep gravity well and has a very thin atmosphere. This makes landing on Mars is challenging. The atmosphere is thick enough to cause problems but not thick enough for useful aerobreaking. Getting useful loads down to the ground is a a technically challenging problem, and getting back into orbit is just difficult enough to be annoyingly expensive.
If only somebody was working on a reusable spacecraft that can use it's own engines to land exactly where it wants to, and then refuel and lift off again. Remember this is for cargo or humans. Before any people go to Mars, I expect to see cargo trips to Mars that will land LOX fuel compressors and robotic follow up that will refuel and launch, with physical samples, back to Earth. Then we'll see some humans go if they figure out all the other issues with moving space habitats that are indepedent for long periods of time.