Why is a package obsolete?

[Garreau, Alexandre]

By giving a quick glance at the emacs manual, I didn’t find how to know
why exactely is a package obsolete in packages.el (I only found that a
package may *be* marked as obsolete, or that you may delete them all):
is it API change from emacs (isn’t it very stable and somewhat
backward-compatible most of time?), a dependency that’s no longer
available, the presence of a newer version that’s not incompatible? or
this last one, for a dependency?

Is there an automatical way to know why was a package marked as
obsolete?