The Cowboys need to give themselves a chance to win while Romo is still playing. Drafting a QB early would be a significant negative impact to that goal.
Draft picks are a salary cap management tool. You have to get as much production as possible out of draft picks during the initial 4 year rookie contract while they have low salaries in order to manage the cap. It's highly likely that a QB drafted in 2014 would be on the bench for at least 2 years.
When trades are made during the draft, current picks are worth a round higher than the next years pick. The 1st pick in round 3 of the 2014 draft is basically worth the 1st pick in round 2 of the 2015 draft or a pick somewhere in the 1st round of the 2016 draft. If you take a QB in the 3rd round and he does not play until 2016, you're basically paying the equivalent of a 2016 1st round pick for the player; however in 2016 you only have him under his rookie contract for 2 more years, whereas you have the 2016 1st round pick under contract for 4 years with a 5th team option year.
If you carried 3 QBs at any time during 2014 or 2015, then it has cost you even more. That extra roster spot has value. You only have a few truly developmental players on the roster each year. Reducing that number by 1 greatly reduces the odds of hitting on a good developmental player.
On the flip side, carrying an older veteran backup QB is also has a negative impact on probabilities. If you can find a young NFL free agent QB or later round draft pick to immediately be the #2 QB, then you have given yourself a chance that player develops and is ready when the starter is gone. Having Orton as the backup is both expensive in cap dollars but it also gives you no chance that the