Why? Simple: You cannot trust it. You need to be able to trust such software. I'm sorry, but you just can't in this case.
I won't go into all of the details, links and examples here. Do a little searching and it should not be too hard to find hordes of people who have lost calendars, contacts, notes, photos and other valuable data thanks to Apple's wonderful software.
I have personally experienced loosing all of my contacts, appointments, calendar items and notes. Of course, being that this ain't my first rodeo I had backups of the backups. I always go into data storage situations with zero trust for what is being offered. It took some work to get the data back but I recovered.
That is not the point. The first three rules of any piece of software that users entrust with their data are:
1- Thou shalt not loose user data
2- Thou shalt not loose user data
3- Thou shalt not loose user data
Go ahead, Google it. The stories are horrific. From people loosing contacts to students loosing semesters worth of notes. Auto-magically.
I even went to my local Apple store to consult with a "Genius" about this. The guy was clueless. And, in fact, right in front of my eyes, he proceeded to accidentally delete all of my calendar entries --something that should be darn-near impossible to do. Again, I had my backup at home so I didn't blow a fuse. But, please.
I explained that it was not reasonable to blow away someone's data if their email service provider changed. If you go from Google to Yahoo all the shit that was being synchronized with Google goes poof. Really? Why? I would love to hear from someone at Apple on this. Why it is even remotely reasonable to blow away all of my contacts, notes and calendar information?
If you close your email account and you are no-longer able to log-on it seems iOS will attempt to synchronize, fail and then blow away your data. My wife has had cases of constantly disappearing notes and appointments due to failed synchronization sessions.
The greater point is that you should not have to be a programmer or IT worker to have your personal data be safe on iOS. She is not a technical person and without my help she would have descended into data loss hell. Grandpa and Grandma have no hope.
Anyhow, now iCloud and all forms of automated synchronization are turned off on all of our devices. Manual backup is still the only way you can be sure your data will not be lost.