All airlines, including Southwest, are masters of deception when it comes to advertising and communicating their policies.You always have to check their websites for details. This is a quote from Southwest's website:
Rapid Rewards Points can be redeemed for any available seat on any Southwest Airlines flight. The number of points required to purchase a reward flight on Southwest Airlines is based on the advertised fare for the fare product purchased and will be available through the flight booking process on www.southwest.com.
I interpret this to mean that a) the flights were completely sold out, or b) the lowest fares, using the fewest reward points, were sold out and you didn't have enough miles in your account for a higher fare category.