The main reason is compatibility, especially with application software, which may depend on specific behaviors of the versions of the system software included i

The main reason is compatibility, especially with application software, which may depend on specific behaviors of the versions of the system software included in the original distribution. “Stable” here refers to the stability of the API and ABI, not to software reliability in general. Many customers would rather pay a small share of the cost to backport only the most necessary changes, in order to reduce the need for continual re-testing and updating of their applications. Keep in mind that many applications are not commercial or open-source packages with many users, but instead were developed for only one organization, which must bear the full cost of maintenance.