They did in a sense. The core cooled, stopped spinning, the planet lost its magnetosphere, and without it the atmosphere fell victim to the sun's rays.
There is also talk that Mars is exiting a Martian Ice Age that is analagous to Earth's. The frozen CO2 at the poles is enough to increase pressure by up to x10 and will likely happen over hte next millenium. There is more and more evidence that the Martian climate mirrors our own as a function of the sun's output. The only difference is that it's Mars atmosphere that freezes when it has an ice age. Who knows, maybe once it does warm up more and the atmosphere sublimes from the poles, the water suspected to be as permafrost may be liberated. That would be cool!