I don't think there is any official story.
But some people believe that Imagineers brought their ideas to Universal after they were shot down by Disney (which doesn't make sense to me, because I would imagine Disney has very strict contracts with their employees, and everything the Imagineers come up with is Disney's property).
Others say that Beastly Kingdom and the Lost Continent were indeed two seperate things, but when Disney caught wind that Islands of Adventure would also have a land dedicated to the mythological creatures (and that both parks would open within a year of each other) Disney decided to scrap the Beastly Kingdom idea because they didn't want to seem like they had the same thing as Universal.