There's only one reason for discontinuing a model: Poor sales. In the case of a model such as the TU250, there may be enough of them in warehouses that Suzuki will stop importing them for the 2010 model year, and bring them back in 2011 if they sell out the back stock. This is practical to do with a model such as this, because it is unlikely to see any significant updates from year to year.
A 500cc version is unlikely: It would have to be a twin-cylinder, and Suzuki already has the Gladius in the lineup.