The reason for the common use of the three-letter acronym, בס"ד (BS"D), is probably because it does not contain the Hebrew letter Hei ('ה), that is used to imply the name of God, and for this reason, a page which contains the letters בס"ד (BS"D) without any other Torah content does not require Genizah (a process for writings that contain the name of God), and thus can be thrown away without a fear of desecration.
Genizah |
With the support of Solomon Schechter they made several more trips to the Middle East, locating the majority of the Genizah at the Ben Ezra Synagogue in Cairo.