After you stop breathing, the body no longer can regulate its pH (one of the main functions of breathing is to exhale out carbon dioxide, which is critical for regulating the acid-base balance of the body). With accumulating CO2, the pH drops significantly (like from 7.4 to 6.8, and lower). Most human enzymes are tuned to be most active at pH 7.4, and a drop to 6.8 renders them inoperable. Thus, most of your bodily activities cease, in dramatic fashion. This allows damage to cellular structures to ensue, including your cellular membranes, mitochondria, and nucleus. Even if you could later rectify the underlying “cause of death”, all of that damage is impossible to “repair”.
Edit: I should add that the lack of oxygen is also a major problem, as mentioned above, but also that reperfusion (i.e., the correction of a blood/oxygen deficit) can also lead to significant cellular damage via a number of mechanisms. So, damned if you do, damned if you don’t.