YMMV on whether JKR is a great writer or not (I personally think she's actually quite a good writer, particularly by the end of the series, but I am not blind to her faults), but leaving gaps in a backstory is NOT necessarily a mark of a poor writer.

Since someone so helpfully mentioned Wuthering Heights, above, I assume Emily Bronte is also a poor writer b/c she never makes it clear where Heathcliff came from? Or Charlotte, for that matter, for not giving us Rochester's entire backstory. Or Jane Austen. I mean, what kind of trade was Bingley's father engaged in? How can we know whether or not Caroline Bingley is actually a heartless bitch unless we know whether she is actually just a poor, misunderstood Slytherin who wants to improve her lot and she DESERVES Darcy, damn it.