As some people simply don't want Retina on their machines (myself included... yet).
Plus I doubt Apple could deliver a 21.5" iMac with retina for $1199, it would be fantastic if they could but I just don't see it happening any time soon.
Yes IF we get Retina this year on the iMac it will either be a new premium model or a very expensive BTO option.
But I just don't see it happening this year, still part of me hops I'm wrong.