One of the requirements for HOTP (which TOTP is based on) was that the code should be numeric for phone entry or similarly constrained entry mechanisms.
> R4 - The value displayed on the token MUST be easily read and entered by the user: This requires the HOTP value to be of reasonable length. The HOTP value must be at least a 6-digit value. It is also desirable that the HOTP value be 'numeric only' so that it can be easily entered on restricted devices such as phones.
> R4 - The value displayed on the token MUST be easily read and entered by the user: This requires the HOTP value to be of reasonable length. The HOTP value must be at least a 6-digit value. It is also desirable that the HOTP value be 'numeric only' so that it can be easily entered on restricted devices such as phones.
See R4 at https://datatracker.ietf.org/doc/html/rfc4226 (split across pages 4 and 5).