My advice: Don't write to be published but because you have the greatest story to tell.

Perfect advice, especially because it's so much more enjoyable when you're writing something you're passionate about.

You can use the time you spend writing to also learn the craft surrounding all of it. It's a steep learning curve, but you can learn as you go, which is what most people do, whether that's proper punctuation and grammar, the "art" of storytelling, reading other authors' works not just for enjoyment but also to study how they write, using a writers' site (whether this one or another), where you'll learn a great deal. Over time, you can read books on writing and even take courses if you see the potential value in those. Most of the best writers on this site, including published authors, learned by doing most or all of the above (usually incrementally over time), much of it centered around being a member of one of these sites.

Also, be prepared to write several drafts of your story, although you'll have to decide how far to take that. If you're like most people, by the time you finish the first draft, you'll have learned enough that you'll know whether you need (or just want) to write another draft.

Is it the quality of writing, the uniqueness of the story, or the author’s platform and marketing potential?

All of these and more.
And sometimes you could have the perfect everything but they have a full catalogue and cannot onboard more. Equally if they are about to publish in the same genre.

My advice: Don't write to be published but because you have the greatest story to tell. Writing quality, platform, covers, those can be learned or bought. Market potential shifts every few years, but it's a carousel. Nothing is truly non-market forever