Puzzlements: Why have so few soldiers and Marines deployed?

As we enter the seventh year of the Iraq war (sorry, that name is already taken), I don’t understand why two-thirds of the active-duty Army and three-quarters of the active Marine Corps have just one or even zero deployments to Iraq or Afghanistan.

Check out this chart from a Pentagon briefing:

And just so you have it, here is the whole study from which that slide is taken.