Lack of big running plays has a lot to do with the fact we haven't had a consistent passing game the last 2 years. If you are stacking the box and daring a Qb to throw to beat you, unless that Qb beats you, your defense will close up most of the running lanes in a hurry.
Open up the passing game, where it becomes respectable, and defenses will no longer be able cheat down in the box. Not being able to cheat is going to naturally free up space in which a Rb can be much more shifty, meet more 1 on 1 opportunities, and produce explosive runs.
We all know where our passing offense ranked the last two years....if we can move into middle of the pack I believe we will start seeing a lot more big runs be ripped off.