I'd guess that maybe they have a limited supply of tankers and if they're only sending ships to Mars during a narrow window of time (due to planetary orbits), t

I'd guess that maybe they have a limited supply of tankers and if they're only sending ships to Mars during a narrow window of time (due to planetary orbits), then it's helpful not to have a bunch of tankers sitting idle when they could be busy making trips back and forth from Earth to Leo. There might be practical considerations, too, like if someone is on the ship for a couple days and then realizes that spending months on a ship or living on Mars is just not for them, they can get transferred to the next convenient ferry back to the surface. We're probably overthinking this, though; what order the passengers, fuel, and cargo load is a pretty minor detail that could easily be revised when the plan is closer to completion.