This is due to a fairly new rule change to the MLB Collective Bargaining Agreement, to incentivize teams to promote their talented young players in order to prevent them from keeping them in the minor leagues for six weeks to extend their service time. This rule, called the Prospect Promotion Incentive, is given to players who are Top 100 prospects that make the Opening Day roster.
Within the last few years, the Rookie of the Year award has grown to have more meaning than it had in the past. Changes in the most recent collective bargaining agreement allow the award to bestow a full year of MLB service time on the top two finishers, even if they would have otherwise entered the offseason with less than that.