In 1987, when your audience of bash and awk and sed users was looking for a language to bring those together and were familiar with them... why wouldn't you use them?

I would also contend that given the tools at the time (vt100 terminals without syntax highlighting being prevalent systems) sigils made it easier to write more on a line, provided easier visual recognition (for those familiar with the language) about the syntax, and provided for a more easily written lexer.