There's a great StackExchange answer on this. Basically, both the terrain and weather are bad: "Sites of areologic interest are far apart, its seasonally induc

There's a great StackExchange answer on this. Basically, both the terrain and weather are bad: "Sites of areologic interest are far apart, its seasonally induced micro-climate conditions are rough and pronounced due to somewhat larger surface atmospheric pressure, winds are stronger, dust storms longer lasting, it experiences higher seasonal and diurnal surface temperature variations, shorter days and exposure to sunlight during Southern hemisphere winters, and as a deep impact basin, terrain is fairly rugged, inclined, and often covered with deep pockets of sand.” https://space.stackexchange.com/questions/12158/why-has-no-lander-or-rover-visited-hellas-planitia-on-mars https://space.stackexchange.com/questions/12158/why-has-no-l...