It's all still a basic function of supply and demand. There are a lot of people playing Mordor at the moment earning cards, and significantly less so with games like Pyrite Heart and Final Fantasy IV.
Also, the way that trading cards are set up almost everyone has learned not to buy cards when they're expensive (like when a game first releases) and to wait for the price to come down as more people place their cards on the market. It's almost inevitable given the abundant supply of these cards and the small reward they give that the price of the cards almost immediately tanks to nothing. When they're close to worthless, the people who want it will craft the badge. Most people have figured out that a badge, profile background, emoticon, 100xp, and a coupon just aren't worth paying $5 for. The cards that have retained their value are the ones that are offering some balance here: backgrounds that are worth a lot of money, :weed: emoticons, waifu badges, etc.
It's sad, but the trading card market has always been a race to the bottom. How fast it gets there is directly proportionate to the number of people who own the game.