Looks like there are technical limitations. May be decryption decreases loading bandwidth or something. And individual files can't be encrypted outside of very rare exceptions, such as cinematics and pets. Due to some reasons Blizzard don't like to change game data between patches. May be because they can't rely on spare free space on your disk to hold both encrypted and decrypted files at the same time.
And in case of cloud gaming developers can actually add content without even noticing players. For example they can add new drop with completely new model to some mob and you will never know, that it exists, till it won't actually drop to somebody. Imagine, that you log in some day and you see something completely new! No patches, no anything. 100% surprise.