because other teams prefer trading for guys with lower ceilings
Because Rizzo has his favorites and judging from quotes like this:
While detailing the “next wave” of Nationals prospects, Rizzo mentioned two by name: Alex Meyer and Matt Purke. Meyer, a 6-foot-10 flamethrower from Kentucky, “looks like he’s 11 feet tall on the mound,” Rizzo said. “He’ll be here soon.”
Meyer is one of them. Unless we're getting a player like Gio Gonzalez(i.e. All-Star, young, projected above average starter for years to come, aka players that most teams don't trade) in return I don't see him trading one of his favorites.
