70% of users are not using galaxy s 3/4, Htc one/one x/one s.
Majority of that marketshare is actually low to midrange handsets. Optimizing a good game for all of them is a nightmare.
Optimizing the same for just one target (iOS) as iPhones are not as wildly different apart from mild processor and memory bumps, is much easier.
Also every one who buys an iPhone can run it. 4/4s/5 (even 3gs ran 2012 3D games, not sure about the new ones, i suspect it can't now) are pretty solid in that department.
Just put yourself in a developer's shoes and then see. A developer is also targeting the return on his investment which seems to be better on iOS.