The photo is looking dull and flat. If you look at the leaf to the right it has more blue than yellow and tend to look aqua. Open the photo in your photo editor and you should notice some blue to the right of the histogram. As a matter of fact @Abambo illustration shows the blue clipping on the white segment (on the right).
Rejection reason? Quality and technical issues or Aesthetic or commercial appeal?
Aesthetic or commercial appeal
There are multiple flower pictures in the database so if you want to submit yet another flower picture, it needs to be great!
Quality and technical issues
As there are many flower pictures in the database you need to have perfect picture quality. Besides some possible tweaks in the white balance and in the lightning, your flower picture is slightly out of focus with the blossom, the most important part of any flower picture. You need to get it crisp sharp, even without additional sharpening. That can't be corrected afterwards.
As for other parameters see here:
Unfortunately, I also detected some artefacts probably due to dust on the sensor.