Why do I have to wait 6 months for a follow-up ultrasound after a breast biopsy?

Ask your doctor. If the biopsy was benign, your doctor may want to check to make sure that there aren't any new problems and that things are stable. The best thing is to ask your physician. They know your medical history and have been following this issue.
Out of caution. This depends on the pathologic diagnosis of the biopsy. A 6 month follow up study is occasionally recommended when the biopsy result is benign. This is done to see if there would any change to the lesion that was biopsied. It is typically done as a caution. You may want to check with your doctor or the physician recommending the follow up study.