No other publication in the world would cover BB so they had to come up with their own.
That is not exactly how it happened. Joe Weider was first and foremost a publisher. He published many different types of mags, including physique, detective, gossip, and even beefcake mags. He was a well established publisher before the first IFBB contest in 1948. Then the IFBB only lasted into 1952, and then started up again in 1959. But all through that gap he was still a publisher.
It's more accurate to say that the IFBB was formed to create content for the mags, than to say the mags were created to support the IFBB.