Keeping Romo up is priority #1. If they don't trust Randle in passing protection, then they're telegraphing runs every time he's in there. Murray in there probably kept the defenses honest in respecting the play action and passing in general.
Randle certainly may have much better lateral agility, but I don't buy this top end speed business. 40 times measure top end speed and Randle's was abysmal. I'm talking strictly on everyone's (including the article's) complaint about Murray being caught from behind so frequently. Randle may be a better back, but I doubt him having a big advantage in top end speed is a huge factor why.
Probably not worth bothering, but you'd need to check a ton of factors to see what happened when either of them broke free or got caught. Some stuff like, how many carries did they have prior to that run, who was the last line of defense that was chasing them down and how fast were they, how much space did the defender have to close before reaching the runner, how healthy were both the runner and the chasing defender, etc etc etc.
Humans just eyeballing stuff and making assumptions is really never the way to provide strong arguments.