Not to sound like a smartass but....
Ummm...because for SOME reason it took almost sixty YEARS and PRS to realize that they have been putting two round bobbin single coil pups together and putting that round peg into a square hole.
Now the bobbins are the same shape as the opening in the pickup rings. Less dirt and gunk and shards of magnetic metal get attracted and stuck in those open voids, less chance of breaking a coil wire (not that that was a big issue but..)...
They take a little getting used to cosmetically..we ARE set in our ways, but once you get over the initial shock you realize that it's just yet another SMART innovation by PRS that was LONG overdue and it's these little smart improvements that put PRS on the map and pretty much changed the quality of all American made guitars for the better.