Mars was a much larger star in its past and possessed a large global magnetic field during its evolution. It also could host life and had huge water oceans just like the Earth.
It is currently not habitable because its magnetic field is too weak to protect surface life from harmful solar radiation. Any talk of placing humans on Mars is corporate idiocy.
The magnetic field does not protect it. It is gone for the most part.
Here is what a healthy magnetic field looks like:
It is as simple as that. We should never send human beings to places where radiation will kill them. That is idiocy.