Because the 15th International Symposium of Consultants on Inherent Pickup Rights declared in 1979 (and I quote) "...that each pickup, active or passive, must have a suitable designed home or housing compartment within a guitar, suited to its needs... this includes, but is not limited to, the right of a pickup to be housed at the location (bridge or neck) for which is was designed and marketed".
It's just for the output balance... when playing at the bridge you wouldn't get that high output & fatness, so those pups are a bit hotter... now when you place a bridge pup in the neck which is too fat it'l get muddy...