... I can't say for certain, of course, not knowing the specifics of the author, his publisher, his editor, or their mutual relationships. There could be any number of answers. It could, for example, be that the author believes his use of comma splices is part of his style, and that the publisher/editor agrees, and that would be that.

From my own experience as a writer I can offer a different potential perspective, however. Writers, especially writers without a great deal of experience at their craft, can make mistakes, and there are often aspects of their style that are objectively poor or incorrect. I'm pretty sure every writer has to deal with this. It's part of the improvement process. However, not every writer is amenable to constructive criticism designed to improve said flaws. Some writers believe that their writing doesn't matter as long as they tell a good story. I'm not one of them, but it's an easy way for a writer to deflect criticism and keep from having to put in the hard work necessary to improve. Simply claim that aspect of your work is unimportant, and move on. If the author has found success with this method, a publisher may be unwilling to push on the matter. The publisher's primary concern, after all, is that a book turn a profit. If it's doing that, it may not be worth alienating your successful author by insisting he change.

And if the author is successful, it means that people are purchasing and reading the book despite the flaws. The flaws are still flaws, and personally I wish that every writer would work hard to continually improve their writing. But success is hard to argue with. And I know that there are many writers, some professional but most amateur, who insist that they don't need to improve their writing because that simply their style, and style to their thinking is a matter of opinion. It can be very easy to see yourself as a rebel standing up to the stodgy critics of the world, as a person who writes to their own rhythm and doesn't care what's technically right or wrong. I've suffered from that myself once or twice. After all, it's hard to be a writer if you don't have some measure of ego about your ability, or at least about your potential. It's too much work if you legitimately believe you're bad at it.

Personally, in recent years I've grown better at walking the line between knowing/believing I'm good and at the same time understanding that I could be better, and that I will be better if I put in the effort to continuously improve. But not everyone has found that line. And when you have success while still believing you don't need to improve, well, you can perhaps see why it would be hard to convince yourself that improvement is necessary, especially if none of the people you actually listen to are telling you so.

But again, I have no actual knowledge of the specific situation in question, and wouldn't venture to say that anything I've written above is applicable.