Complexity
Laboratory forced-draft systems are highly calibrated instruments. It is far more than merely a fan that pushes air around. It is a technically advanced system which is designed to measure the temperature within the entire unit. This means that they are calibrated to have a single temperature throughout. The constant air motion makes this happen. However, it is a complex machine that requires skilled personnel to always be on hand in case of failure.
Maintenance
Most forced-draft ovens are complex. They require a single mechanism of cooperation among the oven space itself, the thermometer, exhaust and heat source. All must be in complete and total synchronization. The slightest deviation can send the entire apparatus into chaos. The result is that they can be expensive and a constant worry for maintenance issues.
Regulation
These ovens are regulated by the U.S. Department of the Interior and similar bodies both at the state and county level. Ovens that are used to dry certain samples of relevance to a crime scene, for example, are tightly watched by various government bodies to ensure their effective operation. In some cases, some samples need to be dried or dehydrated before then can be effectively analyzed. In areas of sensitive production such as nuclear energy or criminal procedure, governments closely regulate these ovens. The resultant inspections increase the price of use, and regulations governing their production also increase their retail price.
Energy
Under certain circumstances, the forced or mechanical draft oven wastes energy. This is because the entire interior space of the oven is heated to the exact same temperature throughout. This is the case even if the sample you are dehydrating is minuscule. In other words, a large oven with an interior space of 3 square feet, for example, would have to be heated to the target temperature for the sake of a single sample. The point is that these ovens sacrifice efficiency for regularity and uniformity.