Why exactly are there so few scientific charlatans?

It's called Peer Review and that what assures that a "scientific charlatan" is an oxymoron or is at least very rare.

A classic example of an exception is Piltdown Man a major embarrassment for the Science of Anthropology in the early 20th century.

In addition to MilesWest's excellent and correct answer, some other factors that aid in reducing the number of scientific frauds are:

A good scientist is skeptical and will generally adhere to the concept of Occam's Razor.

A good experiment will have control groups and be double blind, helping to reduce the chance of self-deception, biased or expected results before it even gets to the peer review process.

Other examples of scientific fraud (either intentional or unintentional) include:

N-Rays

The Canals on Mars

Cold Fussion


Click here for a good "Scientific Baloney Detection Kit."

I don't mean to be chauvinist, but I think the heyday of scientific charlatanism in the last century was in Soviet Russia. The reasons:
(1)the state interfered with the process of peer review
(2) the state interfered with the communication of scientists in the U.S.S.R. with the world-wide community of scientists.

If you tell a scientist what he is supposed to find in his research, don't expect the research to be very fruitful. If you tell the community of scientists that their results are not in accord with your political or other preconceived views, and are therefor false, you will stir up a hornet's nest. It's all about method.