Our awareness of the world does not include smell in the same way as other senses are present. Most of the time our consciousness does not include the odours that surround us, even if we try to focus on what we smell. It is just not there to experience except if there is a really significant change in odour. For example, we know that houses have their own particular, individual smell but we only are aware of it when we enter the house. A few lungs full of air and the awareness disappears.
Nothing evokes a memory as effectively as an odour. But those memories seem to be of a place, a condition, a situation but especially a place.
Smell does not carry the identification of a moment in time, the way sight, hearing and touch do. Smells timing is like a smear, naturally so, for the odours linger long after their creation. It is about a place not an event.
It is possible, of course, to learn how to be aware of odours. Perfumers and wine tasters have each done it. And they have developed a vocabulary in which they can discuss and compare odours. This does not come naturally; professional noses have been trained.
The lack of awareness of odours does not imply that they are not important sensory input. This is illustrated by the paper, Subliminal Smells Can Guide Social Preferences, Dec 2007 issue of Psychological Science. Here is part of the Daily Science item on the paper:
The acute sensitivity of human olfaction tends to be underappreciated. In general, people tend to be dismissive of human olfaction and discount the role that smell plays in our everyday life… Our study offers direct evidence that human social behavior is under the influence of miniscule amounts of odor, at concentrations too low to be consciously perceived, indicating that the human sense of smell is much keener than commonly thought.
So we have a relatively good sense of smell. Not as good as dogs by a long way, but much better than we think we have. We are just less aware of it then we are of our other senses.
This lack of awareness is probably because of the way smell is processed in the brain. All the senses, bar taste and smell, enter the thalamus before passing on to the cortex. The sense of smell does not go directly to the thalamus but directly to the cortex. Not only that, the cortex that it does enter is slightly different from most of the cortex (more ancient and simpler).
The route through the thalamus is probably important to the feedback loops that I assume are basic to consciousness: important to the massively parallel overlapping feedback loops that stabilize on a model of the world.