Why unemployable?
Illegal immigrants are only employable now because they are willing to work off of the books and for below market wages. They are willing to do this because they will starve otherwise.
If they become citizens then they are fully elgible for welfare and subject to other labor laws. They will also demand benefits like health insurance that they are afraid to ask for now since they are illegals.
They will become more costly to employ so businesses will be better off either employing high school dropouts or importing new illegals.
We are then left with 12 million unemployable "citizens" that will increase the welfare load and pad the number of demcoratic voters.