Cost is the monumental reason. Start with a design that can produce 1G from spinning with a suitably gentle gravity gradient that people can live in it. Then

Cost is the monumental reason. Start with a design that can produce 1G from spinning with a suitably gentle gravity gradient that people can live in it. Then try to figure out how to accelerate that very large structure so it can get to Mars. (You can use the fuel as a radiation shield, since there will have to be a metric assload of it.)