I'm not sure of this, but I assume that companies that give you more storage for free or at low prices, are hoping you will operate within their realm. And I think it works.
I have an iPhone6, 2 iPads, & a mac desktop. I love my Apple products, and their hardware works very well. Their operating systems have been dependable, although I think Yosemite and iOS 8 have been problematic. But I pay extra for that Apple hardware. I would think that Apple would like to keep me in its' realm by offering more storage. It clearly is not the case.
Honestly, I have no idea how the iCloud works. It seems unnecessarily complicated. And a meager 5GB for free is basically stingy. Also, this situation where if you take a picture off the phone, it is deleted from the cloud. What the hell is that?
I have traveled outside the Apple world. I use Google+ to store my photos. I have Prime, and can store unlimited photos. I also back up my photos to Flickr. Here's the rub. I do find myself using Google and Yahoo services more because of this. Google Docs, Gmail, and Google+. Yahoo is a secondary email service, and Tumblr is something I use at times. Google just gave me and extra 2GB of storage for free just for making sure my account was secure. That makes 17GB free, not counting photos.
This takes me away from the iCloud, Pages, Apple mail (if that's still a thing), and possibly other Apple services. There are times when I wonder why I'm paying a premium for a phone from Apple, when I operate so often outside their world.