They're fantastic. I think either with the economy people can't afford them anymore, or had big upgrading plans and can't afford to do that any more. Who knows. I think they're great pumps, and most if not all owners of them think they're great as well. I personally would buy Tunze 6100s if I could, but only because you can buy a pair of them them used with a controller for around $400-$450 shipped, versus a new Vortech with a controller for $420 each. The vortechs are better apples to apples, but for the price, I'd be willing to settle on the 6100s if I could afford them at all.
I have two of them and cant complain one bit IMO they rock. But Reefwreak is right they are pricey, and I also suspect this is why you see them FS. They keep good value and when times are ruff people look to sell what they can get top dollar for without selling the ship. Upgrades are usually the first thing to go when down sizing or clearing out and trying to get some quick cash.