The arteries, and the rest of the body, are diseased from the toxic effects of the “free” vegetable oils and animal-foods (meat, dairy, eggs, fish, etc.) consumed by affluent people.
Conjecture.
There are a number of theorized causes - picking out two (ie animal food and vegetable oils) and saying those are
THE cause is an oversimplification.
One is vegetable oils (poly-unsaturated fatty acids) and the omega-6 fats in them. Most people seem to agree that this is a big problem and there appears (at least at this moment) to be little argument that vegetable/seed oils are at least part of the problem.
Another theory is refined sugars. Again, a high probability that this is at least part of the problem. Nobody is suggesting that you eat
more refined sugars...
Another theory is gluten. Definitely some interesting research just coming out about immune reaction to gluten even in non-celiac individuals. Possibly a big culprit in the rise of auto-imune disorders, and possibly a significant contributor to heart disease.
Another theory is lack of anti-oxidants. Anti-oxidants are clearly important for overall health - again, possibly part of the picture.
Another theory is inflammation status and lack of inflammation reducing foods in the diet. Inflammation is a big part of heart disease, and without inflammation you don't the the hardening of the arteries and plaque formation that causes clots and heart attacks. Harder to trace this back to a single thing to avoid/seek out. Inflammation status can be affected by a variety of things including diet, exercise, exposure to toxins, and even things like suffering from gum disease...
Another theory is meat and animal products. Almost everyone seems to agree that factory-farmed, grain-fed meat is not particularly good. There are some camps that say ALL animal products are bad, there are some camps that say that pasture-raised or wild-caught animal products are neither good nor bad, and there are some camps that say that pasture-raised or wild-caught animal products are in fact very powerful in reducing heart disease. Each camp seems to have some decent evidence. At this point, no camp is "right".
Another theory is acidification of the blood stream (acidosis). This also seems to be a part of the problem that most people agree with, however the exact causes of acidosis are varied and its impossible to pin it on one cause. In general, eating less grains, less sugars, less grain-fed factory meat, less processed foods, etc. all seem to help reduce acidosis.
Moral of the story - there are lots of potential causes of heart disease - the complexity is enormous. Anyone who purports to be able to tell you
THE cause for heart disease is highly untrustworthy and should be ignored. Nobody really knows what percentage each of the potential factors is contributing to "causing" heart disease, so admitting that is the first sign that someone is well informed.
The real problem is that people generally already have heart disease before they start trying to do anything about it...