Unfortunately, quite a few of the prospects who are considered top 5/10 in all of baseball will end up being nobodies because of the difference between the competition levels of High-A or AA up to the majors. Dom Brown is a perfect example.
In relation to the question above, I wouldn't say that they necessarily are waiting any longer with their prospects than others. Because so many guys get drafted out of high school, it's difficult to compare them with guys who are getting drafted at 21 and have played in the NCAA, and then you've also got the guys who come from the Caribbean who can come in from as young as 15/16 years old. You've also got guys who play positions that need more time before they can come up and play at a high enough level defensively (i.e. middle infield, catcher). Pitchers rarely come up early because of issues with command, pitch count, pitch disparity, experience, etc.
Having anybody come up to the bigs before they're about 22/23 in baseball is pretty uncommon.