I don't know about other people, but for me programming includes some googling, checking documentation, switching to terminal... basically a lot of alt-tabbing (and some copy-pasting). Can't imagine myself doing this comfortably on a tablet.
And while iOS won't give you a local terminal, remote servers are in reach with a pretty nice SSH client: https://panic.com/prompt/
Text selection on a touchscreen still sucks compared to a mouse, unfortunately.
Still, if an IDE can hit most/all of your needs, I can see iOS working pretty well as a programming environment. Apple's Swift Playgrounds in iOS 10 feels like a first step toward them bringing Xcode over.
Everything about your code, the libraries and APIs you're using, their documentation, function signatures, return types and codes, all a simple keystroke away, without having to so much as glance away from the line of code you're on.
And your code sneakily compiling itself in the background and reporting back anything that could potentially raise itself as an issue, run-time or compile time, presenting it all in the form of little red squigglies and gutter icons.
Imagine writing an entire feature without ever having to alt-tab anywhere other than the end product (browser, app, whatever), and never needing to google anything at all. It'd be pretty cool, right?
That's an IDE. This guy built one that lets you do all that stuff on a tablet.
http://9to5mac.com/2015/11/20/ipad-keyboard-shortcuts-cheat-...
It works the same as on Windows or Mac (not so sure).
edit : I was scooped ! Let me add something : The exact same thing works on Android.