It’s no secret that talent alone doesn’t necessarily dictate when top prospects will reach the major leagues. Ballclubs have significant financial and competitive incentives to keep top prospects down in the minors even when they’re hitting the cover off the ball, or embarrassing every opposing batter from the mound. These incentives are a by-product of MLB’s service time regulations.
For those unfamiliar, the basic concept is as follows: players accrue service time for each day spent at the MLB level, even if they’re on the major league disabled list. After a player collects six years of service time, he’s eligible for free agency.
Things get far more complicated from there, however. MLB has specific regulations in place to account for partial seasons, since the vast majority of players are promoted at some point in the midst of the season. Perhaps the most significant aspect of these regulations (and certainly the most controversial) is that a player doesn’t get a full season’s worth of service time if he spends 12 days in the minors.
Source:: MLB Trade Rumors