I'll tell you why:
1. Julius Jones is a free agent, and he's likely to walk.
2. The league is a two-back league now. Teams like the 1-2 combo.
3. As much as we love Barber, what happens if he doesn't sign or if he signs and then gets hurt? Who do we have as a back up?
4. This is a running-back heavy draft.
5. A running back generally touches the ball more than any one receiver.
6. It's easier for running backs to merge into the line-up their first year than a receiver. It's going to take a wide receiver longer to crack the line up. Look who is going to be in front of him ... T.O. Terry Glenn (assuming he comes back another year), Patrick Crayton, Sam Hurd, Miles Austin, Isaiah Stanback. Even if we get a receiver that is talented, he still has to learn the offense, and I doubt he's going to be better than those guys in front of him. Why not take a receiver in the second or third round considering all that?
7. Who exactly is first-round worthy at receiver in this draft, especially where the Cowboys are drafting? Last I checke, there weren't any Calvin Johnsons or Roy Williams or Larry Fitzgeralds or Braylon Edwards in this draft.
That's why I say draft a running back. As we've seen with Larry Johnson and Deuce McAlister and now Willie Parker, a team best have a backup who can fill in if their starter goes down.
Hope this answers your question.