Antarctica is more suited for human life than Mars.
It is also a lot closer and safer to travel to.
Why colonize Mars before Antarctica?
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One of the reasons why people like Elon Musk want to colonize outer space is for redundancy. If there is a nuclear war or other catastrophe on Earth, it is likely to include Antarctica if it hits all the currently inhabited continents.
They'd have to explicitly include Mars in the nuclear war to destroy it too. With Antarctica, the nuclear winter, lack of trade, and fallout would tend to be enough.
Also, we can study Antarctica without colonizing it. It's not really practical to send small groups of people to Mars and back. So if we want to study Mars, we either do so remotely or we send what is effectively a colony.
Antarctica is part of the Earth ecosphere. This is greatly under strain due to human activity, any large-scale change e.g. to albedo could have wide-ranging and unpredictable effects.
It is quite difficult to enforce preservation e.g. in Europe. Most real estate is owned by somebody, and that somebody might want to pave the field over and build a parking lot, even if that causes rainwater to run off and become a flash flood. Antarctica is claimed by various nations, but there are no masses of individuals yet.
So Antarctica is the one part of Earth where something can be preserved.
By comparison, Mars is almost entirely pristine as far as human influence goes, but it does not have any sentient population. (Or for that matter any significant ecosystems, as far as we can tell. Possibly bacteria deep in rocks.) That makes the case for preserving Mars more an abstract ideal and less enightened self-interest.