Why do upgrades from manufacturers take so long?

Why do Android updates from manufacturers take so long?
A day or two after kit kat source code was released I started hearing about roms ready on some devices from xda. Granted they may not be that stable, but give it a month to perfect or so.

Manufacturers however can take up to 6 months to release an update, and they should have more money and resources to pump into it. Why is this? I'm curious.

Cheers guys!

Sent from my HTC One X using xda app-developers app

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