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Because the manufacturers need time to customize and add their overlay (TouchWiz, MotoBlur, Sense, etc.), then they have to check for compatibility issues. Then they have to push the update to the carrier, and sometimes the carrier will add its own 'fingerprint' (mostly in the way of bloatware). After that, the carrier has to test for compatibility on its networks.
Until Google forces the manufacturers/carriers to use stock Android, this will always be the case. Unfortunately your only options if you want the latest version of (stock) Android are to either get a Nexus device or flash a custom ROM.