Interesting observation! I don't think there's any conscious plan behind it, though.
The 21mm Limited is a very, very good lens. It's small and has excellent IQ. It's sharp across the frame and has great colors.
My theory: the lens is a victim of its own success. The 21mm focal length makes it very appealing for a first Limited purchase, as it's a classic "street wide" on APS-C. However, some of those same people may find that they don't use it as often as they had anticipated. People certainly buy the lens. Speaking for myself, I sadly report that it sits in my camera drawer more than it should. I love it and I use it, but relative to its own qualities, some of my cheaper lenses can be more appealing at various times.
The thing is, if I want a sharp, fast prime for indoor use, I'll probably use my DAL 35 2.4 instead. The extra 2/3 stop of light really does come in handy, plus the "normal" focal length is a good compromise between wide and long. The IQ is also relatively equal between them, as the 35 2.4 is famously excellent.
I also have the DA 18-135 WR, and while of course the 21mm prime is much better optically, the 18-135 is more than "good enough" at 21mm, and it's barely any slower. If I had any of the 2.8 walkaround zooms, it would be even harder to justify the 21mm for indoor use - all those lenses are a touch faster, and as far as IQ goes, the gulf is not so vast among them.
Of course the 21mm is technically better. Of course primes have their own qualities as primes: limiting yourself to one focal length will very often make you a better photographer. Sometimes, zoom lenses can actually slow you down. But, sometimes you really do need that zoom to grab that one shot in a dim room of your cousin falling into a punch bowl, just as you might also not want to deal with your relatives asking you, "how do I zoom this".
I also wonder if some people are disappointed that the wide focal length of 21mm, plus its relative slowness, means that they won't typically get a whole lot of bokeh, unless they're very close to their subject. Many people, especially newbies, are very excited by the idea that a fast prime will give them that bokehlicious "real camera" look. They might not realize at the time of purchase that the 21mm isn't the best tool for that kind of look. They might also not realize at the time of purchase that not every image needs to be shot at 1.8, with only noses in focus.
My point is, the 21mm may have an active life on the used market *because* it's so appealing. It's only later that people realize that it's been sitting in their camera bag for a while, and it's worth a few hundred bucks, and they'd rather use those few hundred bucks on something else.