According to the research report I read, this is because most studies do not attempt to distinguish between first pregnancy abortions and other abortions. It is when the first pregnancy abortions are studied as a separate group that the link stands out.
However, I am mindful of the fact that Saccharin must carry a cancer warning because one experiment, which turns out to have been performed very badly, created a link between cancer and Saccharin. The link is very remote, currently under debate, but Saccharin must carry the warning because we are so concerned about cancer.
The link between breast cancer and abortion is better substantiated than the link between cancer and Saccharin. Why isn't there even a warning that there may be a link? Why is abortion treated as different than Saccharin? Is it possibly because the politics of abortion are more important than public health?
Shalom.