Some major differences, america was an shirtsleeve environment who could support the colonists without future missions. They was also refugees, staying was another bad idea. An Mars mission would require regular supplies like the IIS or an moon base. It would be cheaper to return after 3-6 months than to continue to supply them for decades. You would also want to explore various parts of Mars, doing multiple missions makes this more practical. Yes you save delta-v with an suborbital jump, but hard to move the mars base, better to create an new one, as it's temporary you can use an far smaller one.