Because, what Apple doesn't say clearly is, by doing this you are essentially going to have to pay for iCloud storage. Where before, you had free 5GB for back-up storage. Now, most of that 5 GB will be gobbled up with photos. So its like hey, here's this cool service. But they don't tell you how it impacts everything else.
.....its all explained clearly here:
http://www.pocket-lint.com/news/1314...w-does-it-work
...and also, on Apple's own website (where it DOES indeed say that iCloud is NOT free once you go over the 5gb limit):
https://www.apple.com/uk/icloud/icloud-drive/