<guess type="unresearched"> I'd wager that the investors started to evaluate the actual traffic sources and quality of the candidates going through their system. Upon investigating, although the traffic numbers were high they weren't legit. So rather than trying to right the ship, it didn't have a high enough ROI to save so they just sank it. </guess>

Another article with a lot of good data is http://recruitingheadlines.com/simplyhired-shutdown-latest-n..., and I think the Quantcast stat is worth a lot more consideration than the piece gives.