How come all American Composers Orchestra commissions sound pretty much the same?

Many students have trouble finding their own unique musical 'voice' and most seldom do. However, you really can't compare the efforts of students to those commissioned works by such acclaimed American composers as John Adams, Jennifer Higdon, Bruce Adolphe and Mark Adamo.

John Adams: City Noir:

http://www.youtube.com/watch?v=O-pp6WD89JU

Jennifer Higdon: Violin Concerto:

http://www.youtube.com/watch?v=K9CaIB_HeO4

Bruce Adolphe: An excerpt from his opera performed at the Met:

http://www.youtube.com/watch?v=hnHeQG4g5yk

Mark Adamo: The closing quartet from his opera 'Little Women':

http://www.youtube.com/watch?v=4F_ETuUCchA

(Note: The performance was sub-par in this Boston production but you can't blame the composer for that. I couldn't find the Met production of the same opera but here is a pretty good excerpt from the Opera Mundi (Rio) production:

http://www.youtube.com/watch?v=He_l2dvPhR8

There are other accomplished (and a few renowned) American composers, of course, but these four are favorites of mine. I'm not a big fan of Phillip Glass or Robert Ashley or Robert Morris and the minimalist movement in general.

Because "the safe, technically polished candidate that offends no-one often ends up on top."

because they run out of ideas and borrow from other people, or what they are commissioned to do is all similar in base meaning, leading to similar sounding compositions