iCloud Backup Restore 6.0 to 7.0 but not 7.0 to 7.1 Just for my knowledge and curiosity, not using iTunes backup but just over Wi-Fi:

Why is possible to do an iCloud backup and restore from 6.0 to 7.0 and is not possible to do an iCloud backup restore from 7.0 to 7.1 (they must be 7.0 to 7.0 or 7.1 to 7.1)

It is a software requirement? It is a hardware requirement?
It is possible to restore an iCloud backup from iOS 7.0.x on iOS 7.1. Since iOS 7.1 is later than iOS 7, you can restore the backup fine.