Why Christians change there Doctrine every few years?

Who knows? Depends on the cirstances, really... Christians (and most religions, sorry to disappoint you there) are a wandering circus of bitter, ually repressed gullible thickies, who change the content of the show depending on the venue and the cirstances. Things can turn from litteral to metaphorical to allegorical depending on the scientific progress, commandments into guidelines and back again depending on political cirstances, and of course the enemy changes to suit whatever mood is popular with that specific crowd.