Well its because the spell sharpens the edge of the given weapon, and therefore only slashing weapons.
This is the way the spell has always worked, and if you where to use the rapier for any other form of combat, than its intended, your likely to have to suffer a AB, or it would function as an improvised weapon.
Keen Edge has always been a Slashing only power in DnD. Even if some special functions allows you to get Keen on other types of weapons