Why do I need Wi-Fi to backup to iCloud?
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.