Because they overstayed their Visas into the Bush administration; and Bush kept all of Clinton's immigration policies, which are, by the way, still in effect today!
Do you think that G.W. Could have moved immigration legislation through congress, gotten it passed and signed into law and had it implemented in the Executive Branch, and had every over stayer picked up and deported in nine months. If you do, you are not living in the real world, and ought to restrict your comments to the other patients.