If only a small percentage of miscarriages happen after 24 weeks, then few investigations are needed. That's certainly more manageable than going with the conception precedent.
Stillbirths are much smaller in number compared to miscarriages overall. They're also harder to induce without making it obvious that they were intentional.
Inducing a miscarriage early is very easy and not even likely to be noticed by others if someone wants to hide it. A stillbirth is very noticeable, for obvious reasons.