Anyway, short answer: because they heavily customized and changed a lot of things. They could not have done this with an existing Distro so easy. And sad as it is -> their goal is and was not to help the linux desktop - it is to help the google Eco system. Cloud-Service. Webapp. Makes perfect sense. No matter how much you want them to support the things you (or me) want.

We linux users are not the target group, target group are ordinary newbs who like that everything just runs and don't like that their exe files somehow don't run anymore. But the ones who handle exe files per hand are rapidly declining I guess.