Hi, just curious on what technical aspect caused this photo to be rejected? Thanks!

Nice picture, but seen at 100%, you see that the hearth is not 100% sharp. In addition you see those artefacts, introduced by the denoising. You took your picture at ISO 3200, which is very high for such a motive. High ISO generally has an influence on sharpness and noise. For stock, you should avoid that. 

You also need to know that flower pictures need to be perfectly perfect to be added to the database, because there are abundant numbers of flower pictures. So even if this picture is generally not too bad, it will face more stringent scrutinity as any rarer entry. And sales will be low to none.