The absolute real reason?
They apply your permanent excusal to your record but, over periods of time records have to be updated as people move in to and out of the county. Some agencies just provide the counties with names and addresses with no associated unique identifiers that would tie your new record to your old record. Many counties find it easier and faster to just purge and replace the complete driver's plus voter's lists that they pull jurors names from with a new one and deal with the fallout of permanently excused people having to re-prove their illnesses rather than go through the pain of trying to compare records and make assumptions about who is who.
In my county there is an attempt to do this, but it's not always perfect. Our drivers agency provides us with the driver's license number which allows us to maintain reliable information on permanent excusals, but the voter list we get is just names and addresses. The most flack comes from females who have married and changed their last names. With no unique identifier, there's no way to tell that Peggy Smith who used to live at 123 Maple Avenue is now Peggy Jones who lives on Elm Street.
I could be wrong, but it sounds like your county deletes the old and starts over every couple of years.