Maybe only two people have asked for it here for the DIR-880L, but a quick site search turns up people asking for it for other DIR series routers, such as:
http://forums.dlink.com/index.php?topic=72964.0
and
http://forums.dlink.com/index.php?topic=66000.0
Actually, in that first link, GreenBay42 said of the DIR-868L, "Hmm I know Rev A did have the WPA2 only option. Not sure why they took that out. Unfortunately this product was discontinued a few years ago so not sure if any new firmware will be released."
I don't know how integrated the codebases are for the different DIR routers, but this suggests that this is a feature that has already been implemented, and then removed for some (probably ill-considered) reason.
But I guess I'll just add it to my list of features that I have to double-check before buying my next router. I've recently looked at manuals for several other brands, and all of them include a WPA2-only mode, so this seems to be a limitation unique to D-Link. Yeah, I get that most people won't notice or care, so it's probably not anything that will ever rise to the attention of a product manager. But it's just one more example of how poor D-Link's firmware support is compared to everyone else -- for example, note that the latest released (i.e. not beta) DIR-880L firmware is over two years old.