Mostly, because Dr. Ornish showed in his research that his very lowfat vegetarian diet actually reverses atherosclerosis. And if it clears arteries near the heart, it probably keeps other arteries clear too - like, helping to keep one's brain working well.
And it has many other health advantages as well. Vegans were the only dietary group that had a normal BMI on average, in a number of studies.
The supposedly heart-healthy monounsaturated fat, the fat that's in olive oil, may not be so healthy after all. Rudel did some experiments on monkeys that showed monounsaturated fat caused atherosclerosis just as much as saturated fat. Monounsaturated fat is thought to be heart-healthy partly because it doesn't do bad things to blood lipids, as saturated fat does. But Rudel found that it enables the transport of cholesterol from the liver to the heart, causing atherosclerosis. From some experiments on mice, he thinks this mechanism is probably the same for all mammals.
There are many studies showing the benefits of the omega-3 fats EPA and DHA, though. You only need a small amount of EPA/DHA per day, so it's easy to still have a low-fat diet. I use EPA/DHA made from algae, because I get sick from fishoil capsules.
One gets used to whatever amount of fat one normally eats. I don't find a lowfat diet to be depriving at all. I'm not tempted by high-fat foods. The foods I find delicious are sweet foods. I gradually cut down on high-fat foods, over months.
The high-carb diet gives me a lot of energy, too.