new generations that are higher hashrate and less power use are on the market. and unlike GPU's which they switched over to scrypt coins.. the old gen ASICS get to a point of being useless. so the last ditch attempt to make money is to sell old tech

same applies for gamer's after a while its time to update your gaming machine. at one point playstation 3 was the most sought after item, everyone wanted one but there were not enough to go around. now everyone is buying the new playstation and throwing thier old console onto ebay, cheaper (bt not too cheap to be classed as 'dumping')