The story behind Heritage’s claim begins in the 19th century, with the passage of a sexual purity law that was interpreted to make it a crime to mail or receive items intended, designed, or adapted for abortion. Exactly what the Comstock Act said or meant, not least when it came to abortion, was unclear at the time it passed. States were then, for the first time, criminalizing abortion early in pregnancy, with life exceptions. Members of Congress seemed unsure whether the act covered “lawful” or medically indicated abortion.
In the years since, courts have interpreted the Comstock Act not as a flat ban on all abortions. By the 1930s, courts were interpreting the law as having a sort of implied health exception that applied to physicians and those transacting with them. No one has mentioned the statute in the context of abortion for decades.
Same reason we base our gun laws on a constitutional amendment made when guns were muzzle-loaders.