I've been dealing with vacant homes and their owners for 10 yrs. The most common reason I've found is that the house is usually one they grew up in and inherited from their parents. Thus, there's a lot of sentimental value involved and they simply aren't yet ready to sell.
No, I know several owners that prefer to just carry the property and not have to deal with tenants. They are holding it waiting for the price to go up....it could be inherited or they purchased it as an investment property