Because it is much easier to accomplish in the near term and success begets success.
About half the distance with a much more favorable intercept geometry, about 90% the same G, way less radiation from pseudo magnetic field, 4 times as much solar power available, etc...
Crawl, Walk, Run.
Crawl was the Moon
Walk could be Venus
Run is definitely Mars
You could retrieve surface samples with an unmanned solution and retrieve from the HAVOC vehicle in atmosphere also, landing on any place Mars or the Moon adds so much cost, requirements and risk that it stifles just getting into deep space.
First manned Mars missions, IMO, would be only Mars orbit and maybe Martian moon landings, like Apollo 8 (lunar only orbit) to keep practicing before we play.