It also has the advantage of being in the available unicode set on most machines (because it's a symbol for the Thai Baht, which curiously is divided into 100 "satang"...) The original ₿ started off as an appropriation of the ฿ until a proposal was accepted to add ₿ to unicode:

Thai baht - Wikipedia

So we'd just be repeating this process for the lowercase for sats :)