You know exactly why...because it keeps all the best bodybuilders under the IFBB roof, thus maintaining their dominance over any other BB federation in the world.
It's a rule that was imposed years ago when the IFBB, AAU, NABBA, WABBA, WBBG, etc. were all jockying for position, trying to lure the best to their respective federations, magazines, endorse their products...
It's still a business at the end of the day. Not too different from magazines which have exclusivity, and prevent their athletes from "making a few extra bucks..." writing columns for other magazines. Why should they care? Yes, I realize that the mags are paying their athletes directly...but the IFBB feels that by being exclusive to them, you (as an IFBB athlete) are exposed to more opportunity, bigger prize money, contracts, endorsements, etc...
Hard to argue with the reesults, as more IFBB pro's have contracts, etc. then any other athletes from any other organization...