Because Green is a junior and has had a much more successful college career, same with Wright. Young while probably has more just as much if not more talent than them had a disappointing freshman season and is underdeveloped. That stuff affects your stock.
I agree with you that this is the perception, but check out their college stats:
Young:
14.4 pts, 4.9 rebs, 2.0 ast, 2.2 tos, .478 FG%, .419 3P%, .743 FT%
Wright:
12.0 pts, 7.0 rebs, 2.2 ast, 2.4 tos, .564 FG%, .231 3P%, .613 FT%
Green:
14.3 pts, 6.4 rebs, 3.2 ast, 2.6 tos, .513 FG%, .375 3P%, .775 FT%
Now, when you look at these numbers, I don't really see why Young would be considered to have such a disappointing season, except that Tech wasn't that good this year. It seems like he's mostly just a victim of expectations.
To me, offensively, small forwards coming into the NBA need to shoot and they need to be able to take the ball to the rim. Young is way farther along on these skills than Wright, a sophomore, and Green, a junior. Although he doesn't have a great handle, he can create his own shot and can shoot the college 3. He's right in their league athletically. I throw out rebounding, because small forwards don't really rebound in the NBA.
So when you look at it this way, it doesn't make a whole lot of sense to me why Wright and Green are so far ahead of Young on the draft charts. With Wright and Green, you just have to hope that they're going to learn to shoot, but with Young it's just more of a matter of polishing the skills he already has.