It's just really hard to do new features incrementally on iOS in an app as complicated as ours - on two different occasions in 2015 we released a minor update with a few feature changes that turned out to introduce a serious bug for some % of our users that they then had to live with for a week because of App Review. So every time we flirt with the idea of releasing a few nice features at a time in monthly updates, the realities of App Store development push us back to these big, heavily-beta-tested releases with the endless waits between them.
On Android, if we screw things up we can have a fix out in a few hours, which is why despite running on pretty much the same version of our cross-platform engine code Android is on version 3.2.26 and iOS is on version 3.2.10

, but except when we're doing something Android-specific like Screen Reader we're pretty much stuck following the same schedule so that the two versions maintain file compatibility / (approximate) feature parity / etc.