Using case sensitive names in code within an interactive session is just more error-prone.

Common Lisp is case sensitive. It is just that the Common Lisp reader functionality by default converts all unescaped characters of symbols to uppercase. This is also defined in the Common Lisp standard. The predefined Common Lisp symbols are also all uppercase internally.

Using uppercase was common on old machines. Remember, the design of Common Lisp started in the early eighties (1982) and a goal was compatibility with earlier Maclisp and when there were more types of computers to support (like the so-called Mini Computers and Mainframes). Other programming languages used on older computers also use uppercase identifiers, like COBOL or PL/1.

Also note that Lisp often was used interactively, so that during an interactive programming session getting the case of names right is more difficult. It is slightly easier when the Lisp reader uses a default case (here uppercase) and converts all input to this case.

Common Lisp supports other reader modes and you can also escape symbols: |This is a Symbol with mixed CASE and spaces| .