There are no large organisms that support themselves primarily by preying on humans; so far as I know, there have been none for several thousand years. There are lots of microscopic organisms that do so. Why the difference?
One possible answer is that macro predators face a tragedy of the commons—the deer I don't eat today will not be around, and fatter, next season because someone else will have eaten it. Micro-predators, on the other hand, have an "incentive" to preserve their food supply, both because the bacteria or viruses on me are all close kin to each other and so face evolutionary pressure to act in their common interest and because I am much larger and much longer lived than they are, so that many generations of them are dependent on a single me. From which it follows that a lethal disease is a mistake. From an evolutionary standpoint, diseases want to live off me while doing as little damage as possible.
When I made this point to my wife, she pointed out that some macro-predators solve the problem the same way humans do—via property rights. Their version is territorial behavior. If a single tiger succeeds in monopolizing his chunk of jungle, it is in his interest to let the fawn grow up today to be a better meal next year.
Which leads to an interesting conjecture. Territorial behavior solves the tragedy of the commons only if the prey species is not too mobile—if the fawn spared today is likely, as an adult deer, to still be within the range of the tiger that spared it. It would be interesting to know whether there is an inverse relation between the probability that a species is territorial and the mobility of its prey.
All of which gets me some distance from my original point, which was about humans, not deer. One special reasons macro-predators have reason not to choose humans as prey is that humans fight back. Until recently, we have had no similar ability with regard to micro-predators.
With regard to fighting back, the tragedy of the commons point is still relevant. A prudent macro-predator might hunt humans in a fashion sufficiently selective not to provoke any major retaliation, tiger hunts or the equivalent. But doing so means giving up today's meal for a benefit shared with the rest of his species, making the incentive for prudence a weak one.