Of course there is a certain amount of state and federal red tape, but there are other factors as well. Because many of these tests still take some amount of human involvement, there is a limit to the amount of cases one person can do in a day/shift. Improper processing or simple human error can lead to waiting twice as long for results when they have to be re run. I've only dealt with one crime lab myself, but these people were very thorough and hard working. Every year they hire more people and every year the case loads double.
One other thing that many people are not aware of is that you, as a family member can ask for the preliminary autopsy results. Within a few days from their death you can receive a copy of the physical findings found at the time of autopsy dictated by the Pathologist. They will not give the final cause of death until all results are in, but many times that report will give you an idea of what may or may not have happened, heart attack, pulmonary embolism, that type of thing.
