Algol-60 (and its younger cousin Algol-68) did actually use boldface to distinguish special keywords such as "if", "begin", "true", etc, from the namespace of ordinary identifiers. But that was only used in publications, not in machine-readable source code, since few encodings could support boldface. Instead various stropping[0] methods were used in the implementations of Algol. Today there is really no technical reason why rich text can't be used, but my guess is that syntax highlighting serves a similar purpose without complicating file formats and parsers.

[0]. https://en.wikipedia.org/wiki/Stropping_(syntax)