Because to make good, personalized recommendations they need to know what you don't know, and there isn't a good source of information about it. For example, i

Because to make good, personalized recommendations they need to know what you don't know, and there isn't a good source of information about it. For example, if you have not seen The Shawshank Redemption, chances are you will like it. It's #1 among IMDB top 250 list. But a recommender does not know if you've seen it. If you've seen it already, it's a bad recommendation. So the same recommendation for the same person can be good today and bad tomorrow, depending on something recommender engine does not see. That makes it very difficult to tune and measure performance.