There are three main reasons why:
Reason #1: BAD PROPERTY MANAGEMENT - hiring an ineffective property management company spells disaster for your property. An inept property managers are the number one reason why rental properties fail. Being poor business leader, not being people-person, being disorganized, not being good motivators, not being a problem-solver, and having inadequate information systems - these are a few qualities of a property manager who's not going to be successful managing your rental property.
Reason #2: YOU MADE A BAD DEAL - real estate investing is very attractive and can potentially create wealth for you over time. But many new investors are too excited and didn't have enough knowledge when they acquired the investment. If you made a bad deal, most likely, you didn't verify the facts and information about the property and the area, and you overpaid for the property; and after the expenses and mortgage is paid, there is no cash flow for you.
Reason #3: THE DANGER OF CRUISE CONTROL - Investing in rental properties is not entirely a passive venture. Placing the investment on a cruise control is asking for trouble. There are proceedures put in place to help you reach the goals and objective for the investment.