"Most" is incorrect. Those were illegal abortions and were not always done in sterile conditions, were done by sleazier doctors and in their offices/treatment rooms, often with no assistance. As for the "most," we have no clue how many were done by girls and women alone in their bedrooms/bathrooms.
Back then, women and girls figured they would die during or after an abortion or a botched abortion. Infection was a huge problem.