"Alien" is the term referring to the person. "Illegal" is merely a status.
It's arguable that "illegal alien" is itself dehumanizing, implying that a person can be legal or illegal merely by existing, when in reality, it's the actions they perform which are legal or illegal. In this case, the action is entering the country without following proper procedure, which is more like trespassing, except at a country-level rather than individual property level.
Shortening the label to "illegal" removes the last vestige of reference to the target as a person and further cements the idea that a person's existence can be good or bad.
It's arguable that "illegal alien" is itself dehumanizing, implying that a person can be legal or illegal merely by existing, when in reality, it's the actions they perform which are legal or illegal. In this case, the action is entering the country without following proper procedure, which is more like trespassing, except at a country-level rather than individual property level.
Shortening the label to "illegal" removes the last vestige of reference to the target as a person and further cements the idea that a person's existence can be good or bad.