Well, for starters, some of them are just plain awful.
Although many writers have good stories to tell, they often make two errors:
1) they don’t hone their writing skills through courses, conferences and reading; and
2) they fail to recognize the importance of editing – both critical editing and copyediting. Some don’t even use their software’s spell and grammar checks.
You can be the most creative person you know, but no one can be as objective about their own writing as needed. You need an objective third party (usually not family or friend) to edit your work for you. I’m not saying you need to rewrite the story line (unless your chosen editor tells you the current one doesn’t make sense). I mean smooth the rough edges of how your story is told, make sure your grammar, sentence structure and punctuation is correct, and that your spelling is accurate. Make sure you’ve used words like insure, ensure, assure and the like correctly.
You still may not have a breakout novel. But at least it will be written cleanly and won’t have glaring errors that jar the reader and give critics another reason to say self-published novels are self-published because they weren’t good enough for traditional publishing.
My first novel, Autumn Colors, was published originally by a traditional publisher. When it went out of print, I self-published it. But I was grateful that through the traditional publishing process the book went through multiple rounds of editing, making the writing as clean as it could be. Even with all those edits, one glaring error slipped through – a place where the original first person writing wasn’t changed to third person when the rest of the book was. Still, while I pride myself on being a “good writer”, the editing showed me that even good writers rarely write perfectly. Do yourself and your writing rep a favor and get your work edited before you publish it.