Because a rover doesn't need to eat, breathe, perspire and doesn't have a psychological meltdown after 9 months away from Earth. People are not designed for space travel, currently the technology is too expensive for a manned mission to Mars, we were very lucky the lunar landings went as well as they did and now that the cold war is over the race to put men into space has slowed down somewhat. Robots do the job very well after all.
On Mars...
the mars rover
The Mars Exploration Rover Opportunity landed on Mars during 2004
The rovers were lowered on the planet mars , with the help of a parachute.
Mars Exploration Rover
The first rover landed on Mars on July 4, 1997.
Two rovers of different kinds have landed on Mars. The first Mars Rover landed in 1997. The first Mars Navigator landedin 2004.
The Sojourner Rover landed on Mars on July 4, 1997.
Sojourner, which was the robotic Mars rover, landed on Mars in July of 1997. Sojourner explored Mars for approximately three months. This rover was equipped with cameras and solar panels.
curosity
Spirit Rover
Sojourner Rover