well let's analyze the other bodies... the Sun: what? Moho: tilted orbit, small SOI (smaller than Duna) no atmosphere, overheating Eve: not difficult to land on, impossible to take off, plus there are oceans that can screw up your landing Gilly: minuscule SOI, tilted orbit Dres: small SOI (smaller than Duna) no atmosphere to aerobrake Moons of Jool: require a lot of dV to get to, many have a small SOI and only one of them has an atmosphere, some of them are just a pain to land on, like Tylo Eeloo: easy to land on but requires a lot of dV to get to in conclusion Duna doesn't require a lot of dV to get to, has an atmosphere that can slow you down and help your landing a lot (I landed on Duna only with parachutes many times), has no oceans, has a quite low gravity and it's easy to take off