It seems like a useful ability to be able to detect many different things about, say, another organism. A lot of mammals have this ability. Why not humans? Did we evolve it out or never develop it?
Humans' poor sense of smell is a myth, borne of 19th century speculation rather than actual data. This is described in an excellent recent paper in Science , "Poor human olfaction is a 19th-century myth" by John P. McGann. Quoting from a recent blog post of mine,
The myth dates to the work of the pioneering and hugely influential anatomist and anthropologist Paul Broca, who concluded that the relatively small olfactory bulbs in humans and other primates enabled the occupancy of this space by facilities of intelligence and even free will: “…it is no longer the sense of smell that guides the animal: it’s intelligence enlightened by all the senses… This is the cause of the atrophy of the olfactory apparatus of primates." This presumed “atrophy” and poor sense of smell however, were not based on any actual data on primates’ sense of smell! The myth propagated and expanded in the 20th century, becoming dogma. Back in the 19th, even Freud weighed in, noting that the lack of a strong sense of smell contributed, in humans compared to other animals, to sexual repression and mental disorders.
Humans, when trained, smell quite well, and can for example follow scent trails through grassy fields. Interestingly, the number of olfactory neurons is very similar in humans and other mammals (plotted in the aforementioned post).
There are several factors in human.
we are upright, which brings the nose just about as far away for scents as it can get, you want your nose as close to the ground as possible for the most efficient use scent. This is not in and of itself a deal breaker but it is an important factor which made smell less important for us.
Our large brain is competing for space with our sinuses and nasal cavity. So in turn we have lost a great deal of our nasal cavity as our brain has increased in size, some even speculate this is why we have a protruding nose becasue part of the function of that cavity is thermoregulation and air filtering and the protruding nose helps compensate. The shrinking of our face and snout has pushed this even harder. the less space you have in the nasal cavity the less space you have for olfactory receptors, which reduces the number of distinct smells you can detect.
primates in general have poorer senses of smells than other mammals , they have much better vision however, so the lose of smell was an evolutionary trade off. So humans were starting with a handicap to begin with.
Keep in mind humans still get a lot of information from out sense of smell, but not all of it is conscious. Nearly everything you think of as "taste" is actually smell.
The order of senses from strongest to weakest or more important to less important is basically opinion based. Some would say smell is less important, other would say hearing or some might say taste.
"The sense of smell is not powerful in humans" is absolutely false. They are very powerful as it seems and can be considered as the most powerful sense in human beings. Check this Youtube video which explains why smell is the most powerful sense in organs. What makes sense of smell most powerful is due to its spontaneous ability to trigger and revive memories and emotions. According to this site:
The sense of smell is closely linked with memory, probably more so than any of our other senses. Those with full olfactory function may be able to think of smells that evoke particular memories; the scent of an orchard in blossom conjuring up recollections of a childhood picnic, for example. This can often happen spontaneously, with a smell acting as a trigger in recalling a long-forgotten event or experience.
In addition to being the sense most closely linked to memory, smell is also highly emotive. The perfume industry is built around this connection, with perfumers developing fragrances that seek to convey a vast array of emotions and feelings; from desire to power, vitality to relaxation.
On a more personal level, smell is extremely important when it comes to attraction between two people. Research has shown that our body odour, produced by the genes which make up our immune system, can help us subconsciously choose our partners – read more here. Kissing is thought by some scientists to have developed from sniffing; that first kiss being essentially a primal behaviour during which we smell and taste our partner to decide if they are a match.
Smell is closely related to association and was the first sense to get evolved.
The sense of smell was probably the first sense to evolve in a living creature. In many animal species, smell can play a critical role in finding a mate, communicating with other animals or helping them to survive by finding food. Clearly, though, in humans, smell does not carry the importance that it does in other species.
But the sense of smell is overshadowed by other senses particularly vision because it is most well develop and important sense in human beings.(here, here, here, here) but actually smell is the powerful of them all.
This is a question relegated to evolutionary biology. Our sense of smell is as it is because that is part of our evolution and it is what works best for us. Also consider that you used the word "powerful" The word is arbitrary when comparing the sense to other animals. The more sensitive sense of smell in dogs for example is tied to their survival