Because there are sometimes system configuration changes that dist-upgrade can't manage correctly (such as upgrading the nvidia binary drivers, multiarch changes, ndiswrapper, added/removed architectures and kernel types).
The recommended way to upgrade a Server Edition installation is to
use the do-release-upgrade utility. Part of the update-manager-core
package, it does not have any graphical dependencies and is installed
Debian based systems can also be upgraded by using apt-get
dist-upgrade. However, using do-release-upgrade is recommended because
it has the ability to handle system configuration changes sometimes
needed between releases.