It's easier and much more efficient to get resources such as raw materials and metals from the ground. Being in the air without option of landing means you are completely dependent on import of raw materials to repair/expand your habitat. Very inefficient as well as not economical.

It might be cheaper to get to Venus than to Mars but on Mars you can actually start mining and building factories on the ground. At some point, it becomes much cheaper and more economical. It's just the initial hurdle of getting there with enough stuff and people to start a colony which can self sustain itself and grow.

Once you have brought enough initial stuff to Mars, you get to a point where you can start extracting materials from the ground and free up all the cargo in the ships for just people and perhaps some special items you still cannot manufacture on Mars. But majority of cargo space is freed up.