Because we use a decimal to separate sequences of three digit to make them better readable.
And we don't use a comma for that because the decimal was already in use for making roman numerals better readable, so when we switched to Arabic numerals the most logic thing to make them better readable was doing the same like with the roman numerals, put a decimal between them. And if the decimal is already in use to make your numbers better readable, than you use a comma.

ontopic: ฿, because the B symbol with 2 vertical lines is already adopted by the biggest part of the community as the bitcoin symbol. For fonts 2 vertical lines makes it unreadable, so 1 line is the way to go. And yes it is already used for thai baht, just like the $ sign is also used for more than one currency.