The cowboys aren't set up to run the west coast offense.
We don't have a great pass catching RB,
We aren't deep at WR,
We are deeper at TE,
Our line is better in run blocking,
Our QB is a 7 step drop QB,
Running the WCO would prevent Glenn from running outs and post routes (His bread and butter)
And finally, we don't have enough big targets at WR.
We will run the same offense we have run for years. A play action, power running offense. We run well between the tackles and we pass well in mid range.
The run sets up 8 men in the box which makes seam routes dangerous. Seam routes usually are slower developing than traditional Slants and hooks that you run in the WCO. That's why we use the 7 step drop instead of 3 and 5 step drops. Our line is large and slow, which is good for creating and holding a pocket. We don't have a mobile QB, which combined with our line, means you won't see many rollouts or bootlegs.
Taking advantage of the Seams forces teams to pull up their saftey's into a tighter zone coverage and opens up the deep route for a speedy reciever (Glenn) to stretch the field.
Finally, when the D starts stunting to show 8 in the box before pulling back into zone coverage we run draws to take advantage of the soft interior D.
That's our Offense in order...
Run in gaps 1-5,
Pass around the hash marks and in the seams (10-15 yards),
Take a shot down field on a go route,
Run the Draw,
Repeat...
By the way I take exception to the idea that all the teams you cited really run a WCO. Green Bay is a good example of a "sort-of" WCO. Farve more and more is doing a full drop. Plus they are more of a running team now, then a team that uses the short pass as a run. Even their best WR (Walker) is running mostly routes (15+ yards) that aren't what you would expect from the number 1 in a WCO.
The cover two and various other zone defenses, combined with faster linebackers, taller front 4 linemen, and better bump and run coverages have really hurt the WCO.