There are several reasons. To be fair, people have landed on Mars. But nobody wants to settle there.
The Krasnovians can occupy the Moon by virtue of having a massive orbital Navy to defend it. ISTO effectively controls portions of the Asteroid belt, largely because of their remoteness.
A massive invasion force would take months to arrive. Their launch would be immediately detected both on Earth and even way out in the asteroids. They would also be exposed to any kind of missile or mass driver attack that could be launched from virtually anywhere in the belt. And even if they were overwhelming enough to beat back the ISTO fleets, and all of their defenses, there is the problem of what to attack.
Getting out to the asteroid belt requires a tremendous amount of energy to overcome the gravity of the Sun. However, skipping from one asteroid to another requires much, much less energy. And time. ISTO could detect where the Krasnovians where planning to invade and just move that strategic target out of the way of the invading force. Up to, and including, entire asteroids.
(Okay. Ceres would take a little doing to shift it's orbit properly, but the other points about having advanced warning still hold.)
At present, ISTO can't properly invade the Moon because of Krasnovia's overwhelming military and civilian presence. It could try lobbing Mass drivers at targets on the Moon, but given the extreme range, hitting a specific target is unlikely. There is also the risk that a wild shot could hit something on Earth. (And being a few seconds off on arrival time could be the difference between hitting downtown Franklin and Berlin.)
When it comes to Mars, however, no such compunctions would exist. Any settlement (or other strategic target) is a sitting duck. Can't target that point exactly? No problem: Send more projectiles.
From a non-military standpoint, there are also logistical reasons why a Mars colony is folly. For a colony to be profitable (i.e. justify the steady stream of parts, food, fuel and fresh people), it has to be able to ship something back home. From both the moon and the asteroid belt, shipping material back to Earth is cheap through the use of mass drivers and cargo pallets with ablative re-entry shields. (Though in practice you don't aim for the Earth directly, you aim for near Earth Orbit, and a tractor nudges the material to the final delivery trajectory.)
Because Mars has an atmosphere a mass driver won't work. You need to build a rocket to get to Mars orbit, and from there out of the Mars system and back to, presumably, Earth. And in addition to the atmosphere, Mars has a large gravity well to overcome.
The Moon has a gravity well, but if you time your launches well, you can have the Earth's gravity well provide an assist. None of the asteroids is large enough to have a gravity well that is more than a rounding error to a Mass driver trajectory calculation. And the minor planet Ceres is only a slightly larger rounding error.