A few big differences. There is a huge difference of course between #4 and #10 in the draft. #4, especially in a year where two QBs are picked at the top, means you can get one of the top 2 position players in the draft. FWIW, the draft charts have the value difference between the two spots be a high 2nd rounder.
Second, the Cowboys had signed one good RB the year before, AND had signed another good guy earlier this offseason. The Rams had some guys, but not as good as what the Cowboys have already.
Third, while there are always holes - the Cowboys have a bunch of holes on one side of the ball. And while depth is good, there are better places for the Cowboys to have put their resources than a third RB, even if he ends up #1. Even with their starter getting injured in week 5, they still finished 9th in the league in rushing, after all