I don't know, but I assume it's because they are (were?) trying to "kill" "files" as a concept users are expected to know about.

I wouldn't be surprised if they wanted to force people to create native apps instead of allowing web app file uploading via the browser. Forces extra people to buy a developer license, so more money for Apple. And not to forget, more apps in the store...

Unlikely given that iPhone 1 launched with no SDK and Jobs claiming web apps were just as good. That it wasn't in that initial OS was bizarre enough - that it persisted until iOS6 is really odd.

Just a guess, but it could be that Jobs didn't approve file uploads all this while, and now Cook and team didn't see why they shouldn't allow file uploads.