Because an Android-dedicated framework is of no use to me.

Android will not, in the near future, be a platform I care about supporting more than casually. Game sales tend to be questionable on Android phones and almost nonexistent on Android tablets; the same can't be said about iOS. And while I own Android devices, they gather dust because of my general distaste for the platform's design decisions and goals; while I do not begrudge Android users their decisions, it's become clear to me that, personally, it's very hard to develop for a platform that makes me (literally-literally) sad and unhappy to use.

My primary platform of interest, however, remains OS X and Windows, and I'll consider Linux if I can get it for an expenditure of nearly zero effort. If I for some reason decided that I needed to target only and solely Android for mobile devices, libgdx lets me trivially write code for the platforms I actually care about at the same time.

(But I'm more likely to use C++, where I can target iOS, or MonoTouch/MonoDroid, where I can target both iOS and WP7, instead.)