Checker Finn answers this question in the latest National Journal Education Experts blog :
Despite solemn pledges, statements of intent, and professions of goodwill, in reality most states are loath to set their high-school exit requirements high enough to equal college preparedness--they're afraid too many young people will fail to get diplomas, which is why most current high-school exit tests are pegged at something more akin to 8th or 9th grade curricula.
Read the blog for more of Checker's answer as well as responses from Diane Ravitch, Bill Jackson and Monty Neill