I'm not sure there is a 100% understanding of why diabetes predisposes patients to CHF after myocardial injury. I can't really find a straight forward answer in "Braunwald's Heart Disease, 7th Edition Vo. 2". I think there's a lab at the college I attend studying diabetes effects on myocardium in rat models. Anyway, what I could find (and I didn't really look that hard) was...
Diabetes hinders the compensatory mechanisms which increases the contractility of noninfarcted myocardium post infarc. As time progresses this may render the diabetic patient less capable of adapting to the decrease in stroke volume resultant of infarcted tissue. Factors possibly contributing to this (in diabetics) are myocardial metabolism, insufficient glucose transport, endothelial dysfunction, impaired myocardial blood flow, left ventricular fibrosis and diabetic autonomic dysfunction.
Another mechanism I found is associated with type II diabetes. central obesity-->insulin resistance--> hyperglycemia and hyperinsulemia--> increased cellular lipids, nonenzymatic glycation end products, altered myocardial protein degradation, insulin growth factor mediated effects, altered matrix remodeling, altered vascular compliance, sympathetic activation, and increased renal sodium reabsorption--> maladaptive left ventricular remodeling and left ventricular hypertrophy.
