There are two possible answers, first they want to make sure that they don't jeopardize the players' college eligibility and second, they tend to sit on the jerseys and list for auction just after the draft each year thereby maximizing revenue. It's not a hard policy for them since as unlikely as it may be, Bryce Harper does have eligibly left and could elect to not sign with the Nationals and go back to school. Most of the jerseys they auctioned off last week are from players that have not signed a pro contract. Most probably will but you know how negotiations go, especially for the high school kids that sign just before the first day of college classes.