Why is it so hard for NCO's to Retrain in the Air Force?

If a NCO has reenlisted once or twice and all of a sudden feels like they need a change in scenery or feel like they would be more beneficial in a different AFSC, or just want to learn a different skill set, why is it so difficult to retrain? Where as a First Term Airman, who might not even be sure if they want to stay in past their first enlistment is given the chance to retrain with ease. The Air Force has to pay to send the FTA to more training not even knowing if they are going to get their money's worth before they separate. When you go to ALS(Airman Leadership School) you are usually given a little brown book. One of the sections in there basically mentions how important it is to broaden your career buy volunteering for retraining and special duties. Why does it seem lately that this is nearly an impossible thing to do? I know it is not as cut and dry as this, but it seems like NCO's should be given the same if not more opportunities to retrain.