Mars was probably habitable in the past.
I wonder what disaster befell that lonely red planet, or if Mars just failed to establish an ecosystem able to cushion fledgling life from disruptive events.
An absence of both tectonic plates and a magnetic field could well be the reason Mars has no life today. It's possible that Earth's molten spinning core, which directly powers our magnetic field, is also an indirect reason we have shifting tectonic plates. Both are nice to have, if you're a planet with aspirations of habitability.
The magnetic field protects Earth from cosmic rays that would strip away the upper atmosphere, including the ozone layer that protects the earth from harmful ultraviolet radiation.
The shifting tectonic plates replenish essential nutrients on the earth's surface of geologic time, ensuring that life continues into each new epoch.
So take a moment, look down at your feet, and tell the Earth's spinning liquid core that you appreciate all it does.
(Actually, there's some evidence Mars may have tectonic plates, but they move excruciatingly slow, maybe too slow to do what's required for efficient nutrient recycling.)