I think Apple observed that a lot of people -- not all, but perhaps the majority -- who purchased a 21.5" iMac were doing just what Maflynn noted, purchasing it as an appliance, so to speak -- something they could take home, set up and start using right away for their email, web surfing, iTunes library and that was about it..... Stock RAM, not replaceable? Not a big concern to this type of customer. They are not going to be interested in tinkering with their machine after they've got it home and set up, they just want to use it for five or six years and then start thinking about a new one when the one they have has slowed down significantly, is throwing beach balls all over the place, etc. So......with the retina iMac 21.5" Apple decided to just make the thing something that these people could take off the shelf at the Apple Store and set up at home -- no fuss, no muss......
I have a couple of friends like that, actually, who over the past several years have been perfectly happy with whatever the baseline 21.5" iMac is at the store, and with the size for various reasons......it simply has not occurred to them to want something where they could later upgrade the RAM or do anything else. When their iMac starts showing signs of slowing down their thinking is indeed, "guess it's time for a new machine." One friend is at that point now and another is coming close to it. The 2013 21.5" iMac with the baseline 5200 rpm "platter" drive just doesn't seem to be cutting the mustard any more, and the (I think) 8 GB RAM, ditto......too many beachballs showing up ......
Some of this is how the person uses his or her machine, of course, which is different for everyone. I have the feeling that in a few months I'll be accompanying the one friend to the Apple Store so she can buy the latest retina screen 21.5" iMac with whatever the baseline specs are at that time........