Unlike a human nose, the hand held explosive detector cannot be fooled by other smells that might permeate the surrounding environment. A detector is not a nose in the same sense we as people perceive it to work. Human and animal olfactory glands react to the presence of molecules in the air and the brain interprets that reaction as a smell, usually as good or bad. Animals, dogs especially, have more of these olfactory sensors thus the reason why they can smell things that people cannot. But technology has no brain to teach, so how does it know that there are explosives present? It uses a chemical detector that samples all the chemical molecules in the air.
These sensors are optimised for specific chemical compounds. In the case of the hand held explosive detector, the sensors used are optimised for the chemical compounds associated with explosives. While some sensors might detect more than one compound, it is more effective to apply technology for the exact compounds found in explosives.
Some of the sensors are capable of detecting only a few parts per million of the chemical in the air or on a surface such as clothing. The hand held explosive detector makes it possible for law enforcement personnel to search vehicles, or luggage at airports, or the contents of containers unloaded from cargo ships quickly and efficiently.
You might think that these compounds could also be found in other chemicals or products, and you would be correct. In some instances it might be possible to fool specially trained dogs, but a sensor is that much harder to fool. It does not matter if one compound is overwhelming in smell, a sensor cannot smell. It reacts purely to the chemicals that are present. Law enforcement personnel are trained to know how criminals are trying to fool dogs and technology, and they selectively employ that knowledge to find the explosives.
You will agree that law enforcement might check a container to ensure the chemicals are not used for explosives rather than believe a sensor, only to be killed in an explosion. Thus it makes much more sense to use a hand held explosive detector.