Easy for me: App Store didn't approve the app because it was (apparently) against their set of rules. In Google Play it has been for months and the user base is constantly growing.
For context, what was your app? Were you aware that you would be pushing the App Store rules?
guidelines. The app store has guidelines, not rules. The decision for approval is completely up to Apple. They can approve some dirty porn scam and feature it if they want. Or they can disapprove an app that meets all of their guidelines if it pleases them.
This may be the case now but it is pretty clear that Google is moving towards Apple's curation model.
What makes you think that?
Developers can upload apps to the Play Store without Google's approval