"How did Mars's atmosphere get the way it is?"

The diameter of Mars is slightly more than half that of Earth, and its mass is approximately 11% of that of Earth. It is probable that Mars doesn't have sufficient gravity to retain much of an atmosphere.

Statistics regarding the Martian atmosphere:

Atmospheric pressure – 0.7 – 0.9 kPa
Carbon dioxide – 95.32%
Nitrogen – 2.7%
Argon – 1.6%
Oxygen – 0.2%
Carbon monoxide – 0.07%
Water vapor – 0.03%
Nitric oxide – 0.01%
Neon – 2.5%
Krypton – 300ppb
Xenon – 80 ppb
Ozone – 30ppb
Methane – 10.5ppb

The high level of carbon dioxide (95.32%) probably comes from heavy volcanic activity in Mars' history. There is evidence that it was tectonically very active in the past. One of its features is Olympus Mons, the largest known volcano in the solar system. It's extinct now, and Mars seems to be tectonically dead. At one time it had substantial amounts of water, as witnessed by huge canyons and rills that couldn't have been cut out by anything but flowing water (at least as far as we know; it's doubtful that the Martians had enough beer to do all that). In any case, it's also probable that most of the water dissipated into space as vapor. However, it's hoped, for the sake of future explorers and/or settlers (assuming a lot of terraforming), that much of it is locked up underground as ice.

But before we try to turn Mars into a vacation resort planet, we'd probably better do a few things to try to keep this one from turning into a pizza oven.

Don Firth