"Couldn't they both have something to do with it?"
That depends on whether you believe that tax cuts spur the economy or not and whether you look at tax policy from a dynamic or static perspective. The point is that it is debatable whether the deficit would have been greater or less without President Bush's tax cuts. It isn't debatable whether or not slower economic growth has a major impact on federal revenues.
"It isn't debatable whether or not slower economic growth has a major impact on federal revenues."
Oops! Lost sight of who I was responding to. With YN, EVERYTHING having to do with taxes and the economy is debatable. Let's just say that the impact of slower economic growth on the federal budget is not generally questioned by reasonable people.