For years people have been crying because the iMac design hasn't changed. There wasn't much to change, making it thinner and adding colours were about all they could do. Now people complain that they did change it, it's a lose, lose either way.
My guess is Apple aren't keeping a lot of stock of the colours, with most of what goes to the physical stores being the neutral silver or whatever they are calling it. Online sales are likely outpacing in store sales by a wide margin now anyway, so they can just ship colours as needed. Blue seems to be the default on the website, maybe they got blue paint for cheap, and that's what they have most of.