Its because we've been signing more free agents then we've been losing, and the quality coming in is higher than than those leaving.
Once we get our team roster to a much higher level of quality and depth, then we'll be more likely to get compensatory picks as we have to let some players go in FA. When the cupboard isnt very full, as it was when Wannstedt left, we're in a position of adding free agents rather than losing them.
Remember there was a time when we got 3 compensatory third rounders, so it has happened in our history. Shame we used them to pick Brent Smith, Ronnie Ward, and Derrick Rodgers (although Rodgers was ok for a while).