I suspect because Android has had significant smartphone marketshare for longer and only comparatively recently bumped over iOS in tablet marketshare, so developers targetting Android have been largely targetting it for phone apps. Fragmentation and demographics probably still make iOS much more lucrative for tablet apps (as it still is, AFAIK, for smartphone apps), but the marketshare numbers are such that the lack of tablet apps for Android should start resolving itself, the same way the lack of phone apps did.