Because "illegal" encompases "illegal labor" and "illegal status". In the case of illegal labor, the worker is mostly the victim and thus "illegal" should generally limited to "status". Still, a status is only a matter of procedure and has nothing to do with criminality or delinquency, thus using "undocumented" is prefered to "illegal".
It is very easy to slip from an "irregular status" to criminality in politician lingo. Keeping facts straight helps seeing the reality migrants are facing.
At least that is my understanding.
Jean-Christophe Helary