Because Linux does dominate mobile? Android is Linux and iOS is a *nux. (iOS is a BSD derivative.)
On mobile, Linux is a thin shim of Free software in a proprietary stack.
In revenue (app-sales etc) iOS dominates.
Who made such research? I'm not sure if it's even possible to know exactly, unless all vendors who run their own stores publish sales statistics.
They do, thats one of those things that companies that are traded publicly are required to do.
Publicly traded companies are required to disclose revenue, but not required to break it down by source.
Neither Apple or Google break out the revenue from the *Stores (unless they have started doing it very recently). There are third party analytics firms that provide reasonable estimates though[1].
[1] eg http://techcrunch.com/2014/06/23/google-play-quarterly-app-r...