Maybe its because android is relatively recent. A lot of the more complex apps on linux have already decades of development under their belt.
Maybe its the limitations and/or constraints of the platform with its #ndk and #java apis and various sandboxes.
Or maybe its just a generational thing and those now developing for mobile are less attracted to #foss ideals.
In any case, its a pity because the potential for reaching many people is much greater in mobile than desktop.