Android devices are slow to be updated to the latest version, apart from the Nexus devices which receives updates very quickly. It isn’t the stuff like the custom UI and bloatware that is (unnecessarily) added to the updates, such as HTC’s Sense, according to a Motorola executive.
Motorola’s vice president, Christy Wyatt, claims that slow update times is due to the long time that it takes to write code for specific hardware. She said that:
When Google does a release of the software … they do a version of the software for whatever phone they just shipped. The rest of the ecosystem doesn’t see it until you see it. Hardware is by far the long pole in the tent, with multiple chipsets and multiple radio bands for multiple countries. It’s a big machine to churn.
I can see what she means, as each update for other devices has to be built around the code for the Nexus device. The large difference in hardware, such as processor, slows the coding process down. Networks wanting to “test” the update before pushing it out also slows down the updates.