Landing is really hard. Not enough atmosphere for aerobraking, and too far from home to have enough fuel for a powered landing. It's a fuel problem.
There's the expensive approach. Send multiple unmanned tanker ships into Mars orbit and have them rendezvous into an orbital supply depot. Send landing tanker ship to rendezvous with them and refuel with enough fuel for a powered landing. Landing ship descends. Repeat to build up fuel reserve on Mars surface. Manned ship then docks with orbital supply depot and crew switches to lander. Lander descends, lands near fuel reserve, and refuels for ascent.
Ascent is to Mars orbit and docking with the orbital supply depot. Crew switches back to long-haul ship, refuels it, and goes back to Earth.
There's the expensive approach. Send multiple unmanned tanker ships into Mars orbit and have them rendezvous into an orbital supply depot. Send landing tanker ship to rendezvous with them and refuel with enough fuel for a powered landing. Landing ship descends. Repeat to build up fuel reserve on Mars surface. Manned ship then docks with orbital supply depot and crew switches to lander. Lander descends, lands near fuel reserve, and refuels for ascent.
Ascent is to Mars orbit and docking with the orbital supply depot. Crew switches back to long-haul ship, refuels it, and goes back to Earth.