Why might that be?
Well, research from Karolinska Institutet have discovered that humans are the ONLY mammals whereby no new neurons are formed in the olfactory bulb after birth. Or if there is any, it is VERY little.
In all other mammals including apes, new neurons are constantly being formed in the olfactory bulb which might explain why all mammals with the exception of humans have a superb sense of smell.
How did scientists figure out the absence of neuron growth in humans?
They looked at Carbon-14 levels within the DNA of the olfactory bulb. Carbon-14 levels in the atmosphere increased significantly due to nuclear bomb testing during the Cold War.
As such, IF new neurons were being formed, one would expect Carbon-14 levels which gets incorporated into developing DNA, to be similar to current atmospheric levels. Or at least find a certain turn-over percentage of both "old" and "new" neurons reflective of the Carbon-14 atmospheric levels at time of neuron development.
However, what researchers have found was Carbon 14 concentrations that corresponded to the atmospheric levels at the time of birth of the individuals establishing that there is very limited, if any, postnatal neuron development in the human olfactory bulb.
Although human sense of smell may be weak compared to other animals, it is still the most sensitive sense humans have given it can discriminate more than 1 trillion olfactory stimuli whereas the eye can discriminate several million different colors and the ear only half a million different tones.
The age of olfactory bulb neurons in humans. Neuron. May 24, 2012, 10.1016/j.neuron.2012.03.030
Humans Can Discriminate More than 1 Trillion Olfactory Stimuli. Science 21 March 2014: Vol. 343 no. 6177 pp. 1370-1372. DOI: 10.1126/science.1249168