I think it's just that the bivalent booster was targeting (a) an extinct variant and (b) a different Omicron subvariant. This will target some form of Omicron XBB, I think, which has been the dominant strain for months now.
It's a real chicken-and-egg problem. I've heard people say, "Why should No Man's Sky be on Mac when Mac has no good games?" Well, we can't have good games unless we have good games!