There are numerous unforeseeable events that can occur en route or on Mars. The major concern would be Space Radiation. Space radiation, usually ionizing radiation, has been shown to cause cancer, cataract formation, or death. NASA is developing materials to diminish incoming radiation and utilize a combination of radioprotectors to limit the effects of ionizing radiation. But sometimes the level of radiation will be to high to safely protect astronauts. There are also the effects that microgravity have on the human body. Anyone traveling to Mars would need to follow a rigid exercise routine, and proper nutrient intake would be vital to minimize muscle and bone wasting that would occur in microgravity.
2) It cost too much
The cost of a manned mission to Mars would be 20+ billion dollars. That money could be put to better use fighting the rising national debt, infrastructure, education, and many more immediately useful projects.
3)Robots can do the job
If we send humanoid robots to Mars we can eliminate many of the dangers of space travel. There would be no need to launch food,water, oxygen, or medical supplies in advance. The highly dexterous Robonaut 2 is already being tested for possible missions in low earth orbit and beyond
4)Lack of Technology
Although NASA is developing a new spacecraft for deep space exploration, the technology to lift the required supplies into space hasn’t been fully developed yet. Upon developing that technology there will be an extended period of time when the astronauts on Mars will not be able to communicate with the Earth because they will be on the far side if the sun. Travel to Mars would be feasible after communications, and heavy lift technologies develop further.
5)We have bigger problems on Earth
We have many pressing issues like poverty across the globe, uprisings and wars that are tearing African nations apart, and climate change that is becoming more apparent every years. There are hurricanes, droughts, wildfires, and severe flooding effecting much of the world. Does the glory of traveling to Mars really deserve to continue in spite of the other issues here on Earth?