1. Things depend on one another. Keeping the base system stable while iterating everything else more quickly can end up being infeasible because it's turtles all the way down. Application X wants a newer CMake to build, which wants a newer something else, which wants yet another newer version of something, and then your base system changes underneath you.
2. I really, really love to “fire and forget” a Debian installation for a few years. I've got elderly family members on Linux that are really sensitive to changes in the UI. Things being not changing for years rather than months or weeks is a blessing.