I don't think the 21s are hard to find. There are several in the Knife Exchange now listed.
If you want a certain configuration, condition, blade shape, Box, accessories, price, PPGS, etc then it can take some time to meet all of those conditions. If you find something listed and it is a good price; it will most likely sell right now. If you want a plane jane user it is hard to tell what would be cheaper. The 21 was probably less than the 31 today and you might find them cheaper. Since the 31 most likely has Magnacut the seller might think they can get close to new price.
I was on the hunt for a certain knife and it took some time to get it.
When CRK discontinued the 21 and then went to Magnacut steel then most people seem to be opting for the Magnacut steel (i.e.31). CRK is now selling Magnacut blades for the 21 rather than just using them for warranty only. You might see some people keep them and get a replacement blade.
With the economy right now it seems like some people are selling knives they have not used in a while and that is why I think there seems to be several 21s for sale now. That and some might be trying to get top dollar for some unique configurations.
If you want one (either 21 or 31), be patient and figure out what you are willing to pay and watch the Knife Exchange\for sale by individuals\CRK here.