1. Your Doctor Is Not A Nutrition Expert
I am not sure if you know this, but physicians receive very little education when it comes to nutrition. And I mean VERY LITTLE. I think I had 2-3 hours worth of nutritional education in my medical school. And when I say education, we are talking the very basics - i.e. what are carbohydrates, fats, proteins, what foods they can be found in etc. And the education we do receive is mainly based on the current food guide. We do not get taught nutrition specific to certain diseases, mainly because the time it would take to do this would be immense, and would take away from the rest of the things we need to learn in medical school. If any of you are dieticians out there, you know that it took years to learn what you know in a formalized educational program, and it just isn't feasible to give this education to physicians in addition to everything else we need to learn. In the hospital, when patients have specific dietary questions we automatically ask a dietician to see them to have these conversations since they have the specialized knowledge in this area that we lack. So unless your doctor has a specific interest in nutrition, they are unlikely to suggest anything other than "eating a balanced diet according to the food guide."
2. There is no Evidence
In medicine we always like to have scientific evidence for treatment methods so that we can justify what we are prescribing. And the more studies that show the same treatment outcomes the better. These studies then lead to medical guidelines which are put together by experts in the field so that there is a standard of care for physicians to follow. And not all studies are created equal. The highest level of evidence come from randomized control trials, while the lowest level come from observational type studies or anecdotal reports. When making treatment recommendations we like to have evidence from randomized control trials to support what we are suggesting. As you can imagine, studying dietary changes in a randomized control manner is very challenging. Especially when you are talking about something like the paleo diet. You are not simply asking people to take a new medication, you are asking them to change their entire lifestyle. This can not only be extremely daunting for patients, but can be expensive and time consuming. And as physicians designing these studies we have very little control over what patients actually do in the real world. Patients asked to follow a paleo diet may not actually do so, or may only partially change, and we have no way to control for that. So even if we do manage to study certain dietary changes we usually only get a small number of patients to participate in these type of studies. And the fewer patients in a study, the lower quality the study is thought to be.
When it comes to the paleo diet I have reviewed the literature and as far as I can find, there are only two randomized control trials looking at this diet, plus one other non-randomized trial. One of these studies looked at patients with diabetes and the others looked at "healthy" people and observed the changes. All the studies showed positive results - better diabetes control, weight loss, improvement in cholesterol levels and blood pressure, decreased waist circumference. I have included the links to the study abstracts below if you are interested in learning more.
So as you can see the scientific evidence for a paleo type diet is very limited. And since it is considered such an "extreme" diet physicians feel very uncomfortable recommending it without more studies to back their claims. So even though there is a lot of anecdotal reports and patients who swear their symptoms or conditions have improved by eating this way that is not enough for the medical community as a whole to get behind this way of eating.
I would like to say that this will change in the near future but I highly doubt it. So the next time you ask your doctor for dietary advice please keep these things in mind. I still recommend talking to your doctor before making drastic changes to your diet because there are certain foods that can be very dangerous for people with certain medical conditions, and as you know there is no "one size fits all diet" for everyone.
I hope this was useful! Let me know if you have any feedback or questions.