Apple said it is very easy to update apps, so from a devs point of view why the delay?
Who said it's just the devs? Apple has to check each of the apps before they go through and, with 3/4 of a million apps available, I'm sure that's no easy task either.
From the devs perspective, while the key players knew about the new resolution early, the vast majority of them didn't learn about it until we did - at the iPhone 5 keynote. While it may be a quick and easy fix for some apps, for others it clearly isn't.
A multiplayer, full screen game, for instance, can't simply extend the screen as it could give an unfair advantage and stretching things would make it look distorted. For those apps, they may need to rework much, much more than just the resolution.
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Any good developer will use similar practices to 'responsive design' for web. Meaning, they'll know that no matter what the screen resolution, size or orientation, things will be scaled and laid accordingly.
Since the screen ratio has changed, there is no way to scale something to fit without distorting it.