The primary reason for switching the branch is because of major changes and internal API/ABI compatibility. It's a FreeBSD policy not to introduce changes that would break compatibility for kernel modules into the branch, and that means some things just cannot be backported from -CURRENT. Also, some changes are so disruptive they have little chances for being backported either.
Basically, branches are getting shorter, because more people work on adding major features and introducing major changes, while few people are working on release engineering.