When it comes to habitable planets, among other things, size does matter when staying warm.
As can be seen above mars is significantly smaller than earth.
With it being smaller than earth it has a larger surface area to mass ratio. As a result it cooled much more quickly and to all intents and purposes its once molten insides are now effectively frozen.
Without an internal rotating core the magnetosphere collapsed and there was no longer protection for its atmosphere from the solar wind.
I will not be going to mars because I have no inclination to live inside a microwave oven
Without a rotating core and magnetosphere, aside from loosing its once almost earth-like atmosphere, it is subject to constant bombardment from cosmic rays
Approximately 5 meters of martian soil will be required to produce the same amount of protection from cosmic and solar radiation our atmosphere provides.