Because what Bruce is doing is unmatched. It has never been done before. You can argue that Bruce is "two years" older than Tabata and Martinez, but that is really no where near as true in a baseball sense. Martinez and Tabata are very, very raw, but they also had the advantage of playing baseball every day of their life. That's pretty much what they did every single day until they were signed. Bruce played a lot of ball in Texas, too, but this season was his first full year as well. All went from top high school talent to the lower minors in a year. So baseball-wise, they are very similar in where they are at.
Now, it is just a basic comparison of the stats. Tabata is very impressive, hitting .300 with 28 XBH in 320 ABs at Low-A. He walked 30 times and K'd just 66 times, which is about 20%, which is not too bad for a kid his age. He also showed good speed and good baserunning instincts, stealing 15 bags at 75%.
Martinez put up very similar numbers at Low-A. Slightly more homers, some more XBH, similar BB:K ratio, less speed. The promotion was earned yet questionable because of his age, and it has shown - zero walks, 11 Ks, poor BA. But his power is there, with 7 XBH in 55 ABs. More impressive than Tabata, in my opinion.
Bruce has just been awesome. Similar BB:K rates as the others (42:95, King about 23% of the time), 61 XBH, 15 homers, 41 2B, 5 3B, 18 SBs, .300 average... Everything is there. The power is more than impressive, and the 41 2Bs should easily translate into much better homer numbers as he develops. Great speed, great hitting ability, good patience, excellent defender... He has vaulted himself with this season. This season he is having, at just 19 years old, is really one of a kind. While the other two are excellent prospects and could definitely end up as top 10 guys at some point soon, Bruce has already made the argument and is very deserving of a top 10 spot.
OF (x3): CarGo, M. Bourn, D. Jennings
UTIL (x2): E. Encarnacion, C. Hart
BN: Cuddyer, C. Ross, J. Montero
SP: Price, Gallardo, T. Hudson, Lester
RP: Chapman, Jansen, Rodney, Putz, Cishek, Bailey