Fixing the bug is trivial, but getting the fix out to users is anything but. Google's original model for Android was to have as few restrictions on the manufacturers and carriers as possible. This arguably allowed them to gain a foothold in a market that was starting to be dominated by iPhone, but they're feeling the pain now as the try to lock Android down harder.

Google is moving as much stuff out of AOSP and into closed-source Google Apps package as possible for a variety of reasons, not least of which is that it gives them increased leverage over manufacturers by increasing the amount of work they'd have to do to make a forked AOSP device competitive with official Android. This helps them prevent hostile forks as well as enforcing things like updates and not too much mucking around with the UI. This only goes so far, especially when it comes to compelling updates of devices already running obsolete versions.

Both Google and the manufacturers have very little leverage in practice over the carriers, who seem to be the usual stopping block in delaying updates for various unspecified "testing" requirements.