Based on the season to date, I think this question can be split into two separate questions:
1. Why isn't DD on the field more. I think this is at least partly a question of play calling. If they go to three wides, its going to be Nelson/Jones/Cobb or, when Jennings comes back, Jennings/Nelson/Jones or Jennings/Nelson/Cobb. (IMO Cobb has become such a weapon, especially on slant-type plays which used to be part of DD's bread-and-butter, that he's going to come on the field before DD now.) And Jones is playing like some of us thought he was going to be playing a couple years ago. So that leaves 4 and 5 wides, neither of which is being called as much this year, despite MM's susceptibility to calling too many passes.
What I don't understand is that when they do go to >3 wides, they use Finley as the #4 so often and Driver so little. Which naturally leads into the second question ....
2. Why isn't DD being targeted more. My gut here tells me it is partly that McCarthy and, more so, Rodgers, are more homed in on Finley. Rodgers has always had a bit of a tendency to look to Finley too often, and McCarthy for whatever reason has been calling fewer plays that fit DD's skill set (quick slants, really crisp patterns) and more that fit Finley's.
And it might also be that Driver simply hasn't been able to get separation. Has anyone looked at the film on this? It's just a guess on my part, and would go a long way to answering why it is that Rodgers/McCarthy have the tendencies just stated.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)