Good nerd-sniping question. Just spent the last 5 minutes looking around for an answer, and it seems like the grape/raisin comparison is actually quite apt; both (raisins, prunes) are made from a specific cultivar of (grapes, plums) that is particularly well-suited to drying. Prune juice is made from the same cultivar of plum used for drying, so it makes sense to call it "prune juice" because it's more specific.

-- conjecture below, 0 facts contained --

Grape juice is (I believe) made from different types of grapes than raisins are, so it wouldn't make sense to call it "raisin juice" because you would never make raisins from those grapes.