The Yankees have offered their top prospects in deals for pitchers Cliff Lee and Dan Haren, but have not done so in any deals for Dunn. The supposed reason for this is because the Yankees viewed Lee and Haren as real difference makers and Dunn as more of a bonus than a necessity.
They do have the most potent offense in baseball, leading the league in runs scored. So it wouldn’t make sense to mortgage the future or deal prospects that they could include in more essential deals than one for Dunn.
Bleacher Report - New York Yankees