The obvious answer to this is not having a deep enough QB position group.
Beyond that, though, I think the problem is more a compilation of a bunch of small things just not going right. Some key injuries, sure, but going to back to preseason we had niggling problems that kept the OL from playing together. Vaughan failed to take a step forward and we went with a low-quality backup QB. Randle's head was not right and that led to dithering about the RB position when we should have just named a starter. Dez got hurt, and hasn't been right since. We lost some key role players we were depending on schematically in Scandrick and Dunbar, and that affected the plan overall. Romo going down took away not just our best offensive player, but it also represented the single biggest talent drop off from starter to backup we probably had anywhere on the roster.
The DL got off to a slow start with the suspensions and ankle injury. The LB group had suspension and concussion and ankle issues early. There just wasn't a position group you could point to who was able to get off the blocks at a sprint. Getting Tony back finally and having a shot, and then losing him again. Just the worst.
Sometimes, the accumulation of little things can be a lot more important that single big things. This season has been death by a thousand paper cuts, plus a couple left hooks to the side of the head. The problem is, that stuff can take on a life of its own once it gets bad enough, and that's what I think we're dealing with here. This next game is going to tell us an awful lot about the team and the organization we've got right now.