Scientifically, why does obesity increase a persons chance of a myocardial infarct(heart attack)?

it is linked to hypertension, as well.

The truth is, it's not obesity directly that causes the heart problems. It's the things people eat that make them obese. The cholesterol, the fats, sugars, and so forth that help make people fat also clog up arteries and put a strain on the heart. On top of that, the heavier body makes it tough for people to move, putting additional strain on the heart.

added stress to the bodies system
Obesity makes the heart work harder.
Obesity is linked to myocardial infarction because as the person gets fat..the fats on his body travels through his veins and arteries. For example the vein is a hose, and your finger is the fat. as you cover the opening of the hose, the water pressure increases(water= heart pumping blood) and you when you cover the whole opening of the hose you will feel the pressure of the water. this is true with the fats accumulating in the veins and arteries, covering their openings. the increase pressure of the blood pumping from the lungs would cause the veins and arteries to burst. then death will come.
Scientifically? They probably looked at the stats and autopsies of those who died.