ASCII is a 7-bit code, so there are only so many symbols one can add. Prior to ASCII, you had codes that used as few as 5 bits. Early C had the option to use digraphs to represent various forms of parenthesis, simply because symbols like {} and [] were not universal.

(Strictly speaking, people did continue adding symbols. We used to talk about code pages: when the MSB was 0, the other 7 bits were ASCII. When the MSB was 1, the other 7 bits were a user selected code pade.)