So 100 nonebrities (people I've never heard of) have had their naked selfie photos posted on the internet.
I wouldn't even use the cloud to store my private collection of cat pictures. Any picture I take on my iphone (usually a tree or a daft notice), gets deleted as soon as I get home. Of course, I do then post them on my blog, but the pictures I post on my blog are *intended* for public viewing.
I can't imagine people believing that anything on the cloud is private. "The cloud" means "someone else's computer", and anything could happen, and probably does.
The most likely reason, in my guess, is the common practice of people using the same password for lots of places. They do this because they're advised, or even forced, to choose difficult-to-remember passwords, and everyone knows that you mustn't write your password down.
Actually, I started telling people to write their password down about 25 years ago, and I still do. For example, my PIN number is written down, I keep it in my wallet next to my credit card, but it isn't quite that simple. What's written down is 4829, but I have to add my "magic number" to that to get the actual PIN number. And I have my "magic number" memorised. I also have the combination number for my bike lock written down and kept in my wallet - no magic number is necessary for that, because a bike thief doesn't have my wallet.
And so on. You can easily devise your own way to keep your passwords written down, yet safe.
If you use the same password for lots of places, then as soon as one is compromised, your username/password goes onto the BIG LIST OF ALL PASSWORDS (copies are available via google search) and will get used in password-guessing attacks.
But to put a picture on the cloud that you want to keep private?
That's a bit like using your voicemail without first changing the four digit security code that allows access from anywhere.