Why do I need Wi-Fi to backup to iCloud? What is the rationale behind requiring Wi-Fi to backup to iCloud from my iOS device? What is wrong with using 3G or 4G to backup?
Without going into speculation, there appears to be no official word on why this has been done. There are a number of iCloud Backup KB articles, mentioning that Wi-Fi is the only way to backup your device:


http://www.apple.com/support/icloud/backup/


…however, per almost all Apple documentation of this sort, there is no reasoning to their decisions.

If you need to connect over 3G/4G - you can tether to another iOS device and then backups see WiFi and progress smoothly in my experience. There doesn't appear to be any opt-in choice to self-backup. http://feedback.apple.com would be the only avenue to ask for this feature going forward.