Why do people die sometimes in heart surgery?

Extrememly rare: It's exceedingly rare to die in heart surgery. When it occurs, it's usually because there were complicating factors going in such as severe underlying lung disease and/or generalized atherosclerosis with previous stroke, and/or overall poor health and advanced age. The malnourished or extremely obese, and/or diabetic person with poor kidney function & a weak, damaged heart is at increased risk.

Heart surgery: People die associated with every kind of surgery including heart surgery. There are general surgical risks and specific risks to heart surgery. While surgical mortality is decreasing, likely it will never get to zero. Heart surgery patients are sometimes in very bad shape and carry high risk, these are more often those who die, death is rare in patients with low surgical risk.