Because it harks back to the 1971 model, and actually self-locking nuts were not commonly used on Brit bikes until the mid 70s when nylon became more widely used.
In the early 70s, self-locking nuts were mainly all metal, and were usually made out of round (deformed thread) in order to grip the bolt. Classically this type of nut was not used for applications where oil tight or air tight seals were required.
Because the inch long 1/4” UNC bolt is screwed in from the inside, versus the two 5/8 long bolts being screwed in from the outside, I guess they thought it was important to make sure it didn’t rattle out and obstruct the oil pump or timing chain - hence the split pin.
These days, we’d just use a nyloc or a bit of loctite.
