It's a matter of cost and limited use. The main ingredient of chalk is gypsum, the price is very low, and the texture of chalk is more fluffy, any place is able to write on the word. Talc pen is a common silicate mineral, which is a mineral composition, the later is also the need for further polishing of the cost is very high, talc pen texture is harder, in the smooth and soft places can not write the word, so it is not a substitute.