There may also be other reasons .. such a a third party software author relying on a recent library (or say xorg component) that hasn't or can't be backported to the earlier OS release.
(so the new third party software will never be able to run in 14.04 .. but will work in 14.10 as 14.10 has the newer libraries)
Kinda like software for Win7 that won't work in XP because XP doesn't have the correct DLL versions (and they can't be backported)
There's a lot of "reasons" .. but by far the most common is
no time to check the implications for all other software in the default repos, and known stability/security takes precedence

Third party authors are free to create a PPA and package their software themselves .. but NOTHING goes into the default repos until it's tested, checked and poked
