I think it's a mix of things and part of the problem is the U.S. medical system doesn't seem to know how to diagnose parasite infections beyond maybe some common ones if a blood test is lucky enough to show it.
On the CDC website they even say that doctors in the U.S. know how to diagnose parasites. In other words they are defending themselves cause clearly it is not true.
Some doctors can also be close minded. One parasitologist I might said as if it is a fact that people don't have lyme disease after taking Antibiotics for a month even if blood tests still show it.
There's also another case of a doctor having an easier time making money by taking on simple patients and prescribing medicine and pushing off people who's medical situation is more complicated than the norm. Also the stool tests are a joke which some people think is the main diagnostic tool for parasites. There are some doctors out there who work with infected people tho like on the post above here - //www.curezone.org/forums/am.asp?i=2190036