iOS is much more aggressive about suspending and killing backgrounded apps than Android. Apps are expected to recover from this on their own in order to provide the illusion of seamless runtime, but the difficulty of doing so scales I think superlinearly with the complexity of a given app's state, and between that and most apps these days being more like rich web browsers in that they're obligate clients for a source-of-truth backend, most devs don't seem to bother.