The answer to the question of why people die in police custody is probably second or third on the list of universal obviousness.
Happy, healthy, well adjusted people, are poorly represented in the incarcerated population.
To continue on the trail of the obvious, this means the person who is arrested and put in a temporary holding cell, isolated from other people, is likely to be under the highest level of stress a human being may ever experience. It is difficult for the happy, healthy, and well adjusted among us, to imagine the depth of despair a person may experience at a time like this.
Health, especially mental health is a big factor. It is often the main reason this person finds them self in jail. A small dispute becomes a larger one and someone is arrested for disturbing the peace. Next stop, a small room with no window and no one hears anything you say. Again, it's difficult for the HHWA to understand, but the idea of simply taking off your pants, tying a pants leg around your neck and checking out seems like a viable option.
The process of arresting a citizen can be a physically stressful experience for a person. In the past few decades, we have come to recognize Post Traumatic Stress Disorder as a real thing. Of course, this only applies to people who are doing good things, such as soldiers, public safety officers, and the like. The allowances given to people whose mental state is altered by dangerous and stressful experiences are not extended to people who have been handcuffed and shoved head first into the back seat of a patrol car.
There was a time when the nervous veteran, who flinched when a door slammed, or yelled in anger when someone touched him from behind, was told to calm down and deal with it. Most dealt with it by alcohol or drugs, which numb the senses and slow the reactions. A person such as this, may be at greatest risk when the alcohol or drugs are wearing off, and none is immediately available.
The segment of the population found in holding cells does not get the allowances we give other PTSD sufferers and there is no alcohol or drugs in a holding cell. The effects of minor physical ailments and injuries are magnified. A blow to the head, combined with the expected high blood pressure, can lead to any number of fatal conditions. Blood sugar goes very high in stressful situations. A diabetic person, especially an untreated diabetic is especially at risk.
That's why people keep dying in police custody.