The reason i have read in several places was that John was somewhat dissapointed and offended by not being mentioned in George's own autobiography, "I Me Mine" that was released in 1980. John felt 'forgotten' and it hurt his feelings. or perhaps his ego if that is the way some people choose to view it.
Either way, this was what is beleived to have been one of the main reasons. There may well have been more to it then that.