Because, apart from the L-500 side of the business, the majority of customers have been predominantly Fender players, and Bill has tweaked most of his pickups so that they can be successfully installed alongside single coils with 250K pots. That's the logical reason for tweaking and standardising on 250K, so that all of Bill's pups can optimally co-exist in a 250K environment. Other makers tend to largely ignore coexistence and seem happy for customers to either compromise their humbucker or single coil tones when both are installed in the one guitar. All of Bill's pups are designed to work very well with 500K pots as well, so customers don't need to replace pots when installing into a 500K environment. As to why I believe Bill personally likes 500K pots; I suspect it is because he like things on the bright side. But, it's probably much better to ask him, rather than me. ;)

Aide toi et le ciel taidera (Aid thyself and heaven will aid thee)