It's the modern addiction to novelty. It isn't just fish keeping. A company makes a running shoe or hiking boot that is absolutely perfect, then discontinue it, or worse, change it, to make room for the new year's model. (That one is a particular pet peeve of mine)
Gotta keep selling fish, and that means coming up with the new, improved version. 'Tis the world in which we live, I'm afraid. Those of us who know better can continue to vote with our money, but it ain't gonna change a thing.