Nexus updates are available close to instantly as a download, or rolled out over a few days via OTA. The others have to be worked by OEM's to convert their changes to base into the new system and test compatibility; following that the carriers test to make sure their features work and the experience meets standards and security is intact (apparently they suck at all three of these tests) and then it's released. Apple goes through most of the exact same processes, they just don't release until it's ready on the carriers across the board, so it goes to all customers at once with the same methods as the Nexus updates.