Uh it's because it is an expansion that requires a previous chapter that it had a limited target userbase. Plus the bad publicity from the Hall of Monuments being terrible, the grind for reputation,...and pvpers not having much incentive (given it would be $40USD for 100 skills) makes it an even smaller user base.
The expansion is a fan service, nothing more if you think about it. The fact that it is an expansion requiring a previous chapter is a sure sign of this. (The quote at
http://gw.warcry.com/news/view/74997...e-of-the-North confirms it... "[Eye of the North] is a fan service expansion," Jeff Strain, one of ArenaNet's Co-Founders told us during the demonstration)
When something doesn't sell or is overstocked (probably overstocked in this case) the price is lowered by retailers. I don't think that GW:EN didn't sell well, I think the combination of a small market and a large supply meant the prices had to drop.
The recent patches to GW:EN give me hope that the userbase will expand with time due to fixing a lot of the issues raised since the preview event, however.