Why would it only go back to 1985? The government has been granting security clearances in one form or another since time out of mind.
I’m not sure but I thought individual agencies did their own background checks at one time and that the task was “consolidated” later under OPM.
The article states that CIA has always done their own...
Maybe 1985 is the time that they made the conversion from paper to computer records and the older ones got shredded. The government also has a bad pattern of switching IT systems and a lot of things get lost in the data migration.