Because of duping. Back in 09 sojs were mass duped, so they started using 290's. In 1.10 they started mass duping those, so they used the next best thing: 3/20/20's. Well, you can guess what happened with those... mass duped.
We're back to sojs... just like sov predicted we would be.
As soon as hexs flooded teh game in 09 I told my friend to buy 100's of sojs since the prices went down to like a dollar for 80. And he was like no they'll never be worth nething again. Hm. But really, the currency has kinda been runes all along in 1.10, and those are duped too.