Let's not forget what the heart is, a muscle.

It is the most important of all the muscles because it must operate 24/7/365 without skipping a beat.

To keep a muscle in top condition it must operate at or near the top of it's capacity.

Now look at the modern human being.

How much time in a day do you think the average human being spends at rest as opposed to in some form or aerobic activity, even as simple as walking?

Inactivity leads to the decay and atrophy of the heart. All the rest of the shit that goes along with that- from eating shitty food, to taking all kinds of drugs, to carrying around a couple of extra bowling balls worth of fat for an entire adult lifetime all adds to the problem.

It certainly isn't a high animal fat diet.