Isn't the ATS plagued by reliability issues? or that was an early model problem?
I have a friend that had the CTS (new, I think a 2016) and he is sworn in blood to never touch Cadillac ever again. Got stranded on the highway multiple times to the point where he just walked into a dealership, took the loss and traded it in for a loaded Accord.
That 2017 unsold is a good indication of the market. It will eventually sell, but still, it's odd.