Google has many reasons to reject apps from being published on play store. Apps that do not adhere to Google's play store policy cannot published. While it's completely not clear to reason out, here are some reasons your app is rejected from play store
1. Your app has a serious security issue
2. Your app is spying the users without their consent
2. Your app is not adhering to play store policy
3. Your app is not fully accessible to the testers as part of it requires login