There was an article I probably saw here on HN a couple of years ago. The problem is that rents can't lower to what the market will bear, because that would lower the property value. The loan that was used to buy the property is based on that value, and if the value drops the owner may suddenly need to come up with a pile of cash to make up the difference. It makes more financial sense to leave it unrented at a high price than to rent it out lower.
