Some companies feel that by using Android as a stock OS would be a better alternative to using manufacturers ui’s. This is because it completely delays the updates for your device since you must make the update on top of what the manufacturers have each put into their own version of Android. Google’s CEO believes that by making Android a stock OS would be against the purpose of it being an open source OS. By allowing each individual manufacturer the ability to place their own custom versions of Android onto their devices. By doing so it makes it pretty much completely customizable. I do agree that for updates sake though it would be much easier on the programmers if Android was a stock OS.

Sources: Phandroid, Engadget, Search Engine Land