Why is the therian sign a triangle circle and line?

It's not just a triangle, circle and line. The symbol consists of the two greek letters "theta" (circle with a line in the middle) and "delta" (triangle). Theta is the first letter in therianthropy, and delta often represents change/difference in calculations. The symbol is not an official one, but has been used for a while. It has also gained popularity by being neutral: no species are discriminated by this symbol.