These are my opinions on that.
For one thing they tend to know little about it. It is not taught in medical school, and most doctors are too busy to do much reading beyond the medical libraries, due to all the paperwork and such.
About 1/2 of the average medical journal in these medical libraries is taken up with high priced, full color pharmaceutical ads.
When these magazines do articles on studies etc showing the effectiveness of natural, nontoxic, simple and inexpensive natural treatments, the drug companies get mad as this is competition to their multi-billion plus industries. When they have to pay millions of dollars to get a drug approved, they will lose some business if people use non-patentable things like garlic or an herb that anyone can buy for a few bucks or pick or grow themselves.
Because these things are often so effective, this could harm their industries as many people whose incomes depend on the status quo and business as usual. Thus the drug makers doing the advertising in these magazines that doctors read, threaten and set precedents by pulling out all of their advertising dollars. Without this money which is the main source of revenue, the medical journal often fails and gets put out of business.
Thus when other journals see this, they become very careful about what and which studies they publish..they go for ones that help the drug companies which will bring in more advertising dollars and against things that will upset their clients and risk their own companies. Consequently, most doctors do not know this stuff. Often the average consumer knows more.
Furthermore, when people come in who tried alternative medicine and even healed incurable diseases, the doctors show zero interest in what healed them. For instance, I was told I must take drugs for diabetes for the rest of my life. I refused and when my last few test showed normal, the doctor said "keep doing what you are doing..you no longer have diabetes".
When I tried to tell him the few lifestyle changes I did that led to my tests being normal, he cut me off saying he had no time for any questions or talking, showing zero interest tin learning what healed me that he might benefit his next diabetic patient.
Even my other allopathic doctor of 13 years or so who is extremely into diet and alternative medicine and even sells many supplements and always recommends that and foods, had no positive comment when she saw I reversed my diabetes and lost weight when she discovered I did it my way and not hers...she told me I was on the wrong track..when I tried to say there were medical studies of vegan diets versus the ADA diet that showed people following them did far better than those on the diet recommended by the American diabetic society ( a less than two sentence comment), she become very angry and talked loudly way over me drowning me out as she did not even want to hear it as it was opposed to how she does things (ie--she tells her diabetic patients to never eat any fruit or whole grains the rest of their life --which I consider to be bad advice though I did not tell her I felt this).
She was quite rude and seem very miffed that I dared to did it my way and succeed. She told me to immediately give up fruit and grains forever and then not be a vegan and to eat meat and oils and veggies. Why would I when I had 3 tests showing my blood sugar and a1c completely normal..I am going to continue what works till and if it stops working and then intensify it with more changes I feel will help.
When I told her I did a 50 day juice fast and the fruit juice had no effect on my blood sugar as it plunged to normal every single day I was on it no matter how much fruit juice I drank, she became furious and told me "I told you never to juice fast and to consume no fruits" showing no interest in the surprising fact that the fast did not hurt me as she said it would and the fruit juice had no effect on my sugar nor when I told her every single time I eat meat or cheese, my blood sugar soars to 375..she told me to eat meat.
Being wrong and having a patient not listen infuriated her and she did not want to hear someones experiences did not jive with her predictions...she did not want to learn what could have helped her other patients. This surprised me as I always thought she was open minded and forward thinking and wanted to learn.