Looks to me like at one point it did have reeding. I can see the faint lines in your photo. So, post mint damage.
Kennedy Halves are heavy and the reeding on the edges can get flattened and worn away over time, especially if used at a casino.
It all depends on when, and how, the reeding was missing. If it happens post mint, meaning after the minting process, it's not because most likely someone caused that, possibly as an attempt to dupe someone to think it was a mint error. If it happened as part of the minting process, it could be an error in the minting. If I'm not mistaken, the reeding is a separate process in the minting.
I think you are right BU. Can someone verify this--> The reeding is added to the planchet prior to striking.
I think it's done at the same time the blank is made into the planchet by raising the rim on the edge.
Sorry but you guys are wrong. First the blanks are punched out. Then the blanks are sent through the upset mill to apply the raised rim which turns the blank into a planchet. The planchet has a smooth edge. After being annealed, washed and dried the planchet is fed into the coining press and struk into a coin. The collar die has reeding (if applicable) and the reeding is struck up as part of the striking process.