The MAV needs time to make fuel.
The MAV is pretty cool. Turns out, through a neat set of chemical reactions with the Martian atmosphere, for every kilogram of hydrogen you bring to Mars, you can make thirteen kilograms of fuel. It's a slow process, though. It takes twenty-four months to fill the tanks. That's why they sent it long before we got there. Ch. 1
Ares 4 will be landing Schiaparelli crater, 3200 kilometers away. In fact, their MAV is already there. I know because I watched Martinez land it. It takes eighteen months for the MAV to make its fuel, so it's the first thing NASA sends along. Sending it forty-eight months early gives it plenty of extra time in case fuel reactions go slower than expected. Ch. 7
What if the Ares IV crew gets there and the MAV is bust?
The MAV was soft-landed (as opposed to the balloon bounce-fest the other supplies had). Of course, it was in constant communication with Houston, and if there had been any problems with it, we would have passed by Mars and gone home without ever landing. - The Martian, Ch. 1
What about wind gusts? What stopped the Ares IV MAV from toppling?