The primary reason is . boot9strap uses boot9.bin to:

Technically, boot9 is identical across all retail 3DS consoles (unlike console-unique keys). However, using a friend’s dump is still copyright infringement of Nintendo’s boot ROM. Moreover, some later CFW tools perform checks to ensure the boot9 dump matches the console’s hardware ID; a mismatch can cause a brick.

Understanding boot9.bin : The Cryptographic Root of the Nintendo 3DS