Because we still have documents that were created in the 50's and 60's that are classified, declassifying them would give us a better view on how history is written and why it is done so. Why should "We the people" need to ask the government what it's doing with our tax dollars when they should openly tell us. Knowledge is power.
“Those who control the present, control the past and those who control the past control the future.”
― George Orwell, 1984