I've wondered why iOS kills the last app when it switches away... is it that iOS devices are more memory constrained? Is it that they don't want to disrupt the

I've wondered why iOS kills the last app when it switches away... is it that iOS devices are more memory constrained? Is it that they don't want to disrupt the foreground app by collecting that memory when it's needed rather than immediately? Is it that they want app developers to be more careful about being able to reopen to the same spot they were, perhaps so that the experience of opening the app isn't different depending on whether the app got terminated while in the background or not (or even between device boots)?