Because the only reason they bought the bitcoins was to invest for the interest rates? (I'm not saying all pirate investors were like this, but some must have been.)
Why would anyone think it was the majority. You're not invested in Bitcoin but you decided to take the leap because of an extremely risky HYIP opportunity? I would think that's almost no one.
More like you're invested in bitcoin already & find this opportunity which you like so decide to invest in even more bitcoins. Opportunity is now gone so you sell off the extra bitcoins.
Or you had wanted to sell off bitcoins but didn't because they were earning good interest in this scheme of pirates, but now its over you sell them.
Mind you if the current price is low people might decide to keep them for now. Seeing as the price hit $15 recently it might be hard to sell for half that.