Features - iCloud is just an extension of the stuff that was on iOS already. Previously you had preinstalled apps as part of the rom that could run in the background etc, and you had apps that you could download that ran sandboxed and had to have an icon, etc. iCloud allows direct access to the filesystem remotely. iCloud also now has routing support to assign you to different Apple servers, etc. All of these features would make it trivial for the NSA to put their own special server in at the iCloud data centers and redirect specific people onto it without them knowing.

So while it would have been theoretically possible for the NSA to do it before iCloud, iCloud makes it actually practical to do it without subverting the whole iOS team.