Hansen has command of his fastball, and he proved in ST that he can get out ML hitters. They are fine-tuning his slider, so he can dominate ML hitters. Lester hasn't mastered any of his pitches, and he has not yet proved that he can get out ML or AAA hitters.
Not everyone of these prospects will make it. Of all the arms in the organization, maybe 4 become successful major league pitchers that last a few years. You'd be lucky if two become stars. The rest are washouts or at best journeymen. The FO has to figure out which guys make it and try to trade as many of the rest to improve the team. Which 4 do you think will have major league careers, and which two become stars?