We are all very curious about Mars. Why is it red? Was there ever water on the surface? Is there life on Mars? It would then make sense to send a manned space craft to Mars to answer these questions, right?
The problem with sending a manned space craft to Mars is the fact that it is dangerous. You can argue and say that all space travel is dangerous, and you would be correct. The difference here is that our Astronauts have never spent too much time that far from Earth. The trip to Mars alone is an estimated 6 months each way, and "psychosocial" pressure becomes high in a small group that is isolated for months.
The reduced gravity could also cause loss of bone and muscle, not to mention the fact that outside of Earth's orbit there is an abundance of dangerous radiation particles. Although we have successfully manned 6 trips to the moon, there are still no standards for limiting radiation exposure, and a trip to the moon exposes astronauts to far less radiation than a trip to Mars would.
Scientists are currently working to solve the problem with the radiation exposure, but they have found that they do not have experience with the two main types of radiation that is out there energetic protons from the sun, and the heavy ions in cosmic rays.
Until the time comes that scientists know they can protect our heroes, we have enough training for them outside of Earth, and have proven that we can get by the dangers of traveling so far away then it is best we continue to send probes and robots to mars, rather than people.