Why this picture got rejected?

You should probably get an app that allows you to shoot raw on your iPhone. The iPhone has a quite aggressive in-camera processing that creates nice-looking pictures on the phone screen, but when you look at them in detail, you see the many artefacts. You have also that branch at the upper-right side in the picture. You should photoshop that out. The branches on the right side are also disturbing. You can photoshop them, but it's more complicated. They could also get cropped.

Phone pictures get accepted, but they get vetted with the same stringent criteria than pictures from high-quality cameras. Except if you become a high-quality high-volume stock photographer, photographing what is asked for at the moment, your investment, however, in a real camera won't pay back with stock alone.

If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html