Why are Dark Reavers so rare?

[The]

Dark Reaver is the de facto SK weapon aside from a mith 2hander until raid content. The ratio is only slightly better than a Gigantic Zweihander, but they're both not exactly common. On top of that, the Reaver effect stacks with siphon strength, so a SK can get almost 40 points of str between the two.

The mith 2hander is better hands down, but it's also really out of reach for most people. A reaver is more feasible to obtain, although it seems to be super rare at the moment. I paid way too much for mine, but I like the effect so much I don't want to let it go despite upgrading it already.