The carrier takes the latest version of Android, and they put all of their proprietary code on it. For example, Sprint makes sure their NASCAR, NFL, Sprint Zon

The carrier takes the latest version of Android, and they put all of their proprietary code on it. For example, Sprint makes sure their NASCAR, NFL, Sprint Zone, and Sprint TV works on it, as well as disabling tethering without the plan addition. This is in addition to whatever modifications the handset maker needs to make (e.g. HTC Sense UI, Facebook for HTC Sense, HTC Friendstream, HTC Footprints, etc.). Given all that is required to upgrade to the latest version of Android, sometimes it makes more sense to work on a new phone.