Coda's main competitors back in the day included tools like CSSEdit and Espresso, tools that helped those working on CSS by giving a nice front-end where you could click around in the UI and it'd insert the relevant properties for each style without you having to remember the syntax, and then updating a live preview. All of which seems rather quaint now we have pretty good developer tools in most browsers, but back in the mid-to-late 2000s, that was a big deal for a lot of web devs.
And for web developers who are writing predominantly HTML and CSS (possibly with some kind of template wrapping), a bit of JS, and a teeny bit of PHP/whatever on the backend, "it doesn't support Rust" etc. is not a major problem for them.