The Roadster is built on the same assembly lines as the Lotus Elise (and shares some components like airbags windshield, and some of the interior).
Lotus is no longer going to make the Elise, so that manufacturing partner is going away for the Roadster. It will cost to much money to try and move the glider manufacturing elsewhere in the short term. Tesla is focused on getting their own manufacturing up and running for the Model S, so they are stopping production of the Roadster. I am sure if Lotus were going to continue to manufacture the glider, Tesla would continue to sell them - but it is unfortunate timing while they try to get the Sedan out the door.
Tesla has talked about possibly creating a convertible version of the Model S in the future (sometime after the Model S and the Model X crossover) but they have not set a timetable on it. I assume no real Roadster-style replacement for many years, unless they partner with a different manufacturer to help produce it.