Most likely because it would embarass our representatives to raise the number of visas from, say, 250,000 to 12 million. And the current guest worker programs are just that, come here, work, go home. The illegal lobby is looking for come here, work and stay forever. And bring the family. That's really a question that should be addressed in the context of a comprehensive review of our immigration policies, not emergency migrant labor legislation to legalize the presence of 12 million workers.
That was the point. ;)