Mars allegedly has no atmosphere to speak of because it has no magnetic field, the lack of a magnetic field allowed the solar wind from 143 million miles away to sweep its atmosphere away.
Venus, which also lacks a meaningful magnetic field, and is 76 million miles closer to the sun and therefore has a much stronger solar wind, has a very dense, thick, soupy atmosphere.
How can dat be, eh?
Because..... shut up thats why. Lol