The reasons for the majority of maternal deaths occuring post-partum are probably related to two main issues,.
First, and most commonly, we usually do recognise the woman who is becoming seriously ill and will deliver her prior to death, to both both give her fetus every chance, as well as trying to effect a 'cure,' as delivery of the placenta is the only cure available.
Second, some women develop pre-eclampsia de novo post-partum (no clinical evidence prior to delivery) and other women have poorly observed post-partum deterioration as many clinicians believe that she has been cured as soon as the placenta has been delivered (see above). However, a significant minority of women, especially those with early-onset pre-eclampsia, will suffer a clinical deterioration (BP, platelet count, liver function ...) post-partum that may end up being life-threatening.
Some women simply present with such advanced disease, or having already suffered a life-ending stroke, for example, that any and all clinical excellence will fail to retrieve the situation. This is particularly an issue for immigrant women and women in the US who are without health coverage and who cannot afford to present early with symptoms and do not receive replete clinical care.