Echoing other comments here, but what it comes down to is that the only habitable part of Venus is its atmosphere, which means a natural difficulty in producing

Echoing other comments here, but what it comes down to is that the only habitable part of Venus is its atmosphere, which means a natural difficulty in producing raw materials to produce more habitat. You could figure out some clever way to produce carbon structures from the atmospheric gasses, or even plastics with great patience (there's some hydrogen in the atmosphere). However on Mars you can just build a mud hut for starters, then build up to some native martian concrete and steel. Venus is great for a scientific outpost, but Mars is better suited to a self-sustaining colony.