I know someone in the past disagreed with me on this, but Cross has more of a downhill running style and it's been fairly prevalent in his college career. That's not to say he doesn't have lateral quickness or agility, but, he's definitely a different type of runner than Abdullah. I think that's one reason he's been limited. It's conjecture, but, perhaps the coaches believe Cross isn't the player they want getting significant carries, or else he'd be in there getting them.
I also know a lot of fans want to see Abdullah get less carries, spread the ball around and keep him healthy. It's great to say that on paper, but application is difficult and depends on several factors. Plus, with how well Abdullah is playing, it's difficult to justify giving him the ball anything less than 20-25 times a game, whether that's running or catching. That's a third of your plays right there. Then, if you consider we throw the ball about 20 or more times a game (especially when we had Martinez), that's leaving anywhere from 5-15 plays (on average) to give carries to other backs, run reverses, designed QB runs, option, etc., etc., etc.
I'm sure many of us realize this but it's all important to keep in perspective.