According to Greek Mythology, Calypso was a sea nymph who delayed Odysseus on her island, Ogygia, for seven years.
Odysseus was a pretty tough guy, and happily married (if I recall the Iliad and the Odyssey properly). So, Calypso had to have been one hot nymph and have some mesmerizing power to lure him into her trap.
And, I think that is a fitting description of the Spyderco Calypso Jr, though I would argue Junior tends to not make one think of a female mythological creature.
I've carried a Calypso Jr. of one sort or another for a few years now, off and on. I've tried other knives (the Delica and Native from Spyderco, and most recently I'm toying with a Benchmade 530) but I always come back to my Calypso Jr., most recently reincarnated as the grey FRN model.
I can't say why the Delica outsells the Calypso Jr. Maybe it is the tip-up carry (which I actually prefer on knives of this size, and is my one and only gripe with the Calypso Jr.). Maybe it's the price point. Or perhaps it's strictly availability. The Delica is EVERYWHERE. Any store that carries decent knives has a Delica. I've only once seen a Calypso Jr. in a store, and that was a returned, open-boxed serrated black FRN.
All I can say for sure is that I agree with you, it's my favorite knife too. Except, I think it's a GOOD knife, because it mostly stops me from buying other knives, unless there's something REALLY appealing about them (the Para-Military and forthcoming Kopa, for instance).