My guess would be that it's because (as the above poster says) "app users are much more engaged than website users" and only "half our" moved without nudging - the sites would like more engagement from all users.

That said, the harder you "nudge" me, the more I want to avoid the app and the whole business. Especially if you have any other dark patterns - I will assume you want me to download your app just so you can abuse me better.