cost-effectiveness
While there are some advantages to getting uncontaminated samples from a known spot on Mars, that would cost maybe $2 billion and have a significant chance of complete failure.
Spending $2 billion retrieving meteorites from Antarctica would, I bet, have a higher chance of bringing back one Mars meteorite, may very well bring back multiple Mars samples, and be guaranteed to bring thousands of others - some lunar, maybe even Venusian meteorites.
Sounds like an easy choice to me.
(And maybe the most cost-effective use of money would be to increase the budget for characterizing meteorites that are already collected and are sitting around in plastic bags for years.)