HTC U.S. has posted a massive infographic explaining the various stages a software update has to go through before it reaches the end user. In board strokes, there are five stages: evaluation, development, integration, certification, and push to customer; and in each stage, there may be many steps and other parties (e.g. chipset vendors and carriers) involved that could lead to further delays. In the case of the HTC One, the unlocked, developer and Google Play editions have already completed their update to Android 4.4 KitKat, so no surprises there.
Of course, this only explains how HTC goes about rolling out a new Android release for its devices, and the steps may not be exactly the same for other vendors like Samsung and Sony. That said, we don’t think this is very far off from what goes on behind the scenes for these other vendors whenever there's a new Android version.
If such info is your cup of tea, then scroll down for the lengthy infographic.
Source: HTC U.S.