Why? Before 1990, the mintmarks were added to the completed dies.

When added the mintmarks to the die, the locations/angles/even some mint ID's were not the same because they were added by hand. Thus the locations were a target area. RPMs appeared on different locations, depth of punchings, weak punchings, all these thing varied for years. But since 1990, the locations, sizes of the devices are all the same. Because they are since then on the dies.
I liked the hand punched dies as they made matching up mintmark locations (because they varied) easy to identify RPM (Re-Punch-Mintmarks) and doubled dies because the the mintmark locations. That is why.