Hello,
Firstly, taken with a mobile phone, the picutre quality will never be as good as when taken with a DSLR/Mirrorless camera - due to the sensor size.
Secondly, mobile phones save data in JPG/HEIC (in the case of iPhone), which can lead to compression artefacts.
Thirdly, the exposure in this picture isn't so good for general stock quality.
And of course, composition is a factor. In your photo, not only do you have quality issues but you also have composition issues.
Read these links to get a better understanding of the issues.
User guide:
https://helpx.adobe.com/stock/contributor/user-guide.html
Exposure:
https://www.adobe.com/creativecloud/photography/discover/exposure-in-photography.html
Composition:
https://www.adobe.com/creativecloud/photography/discover/photo-composition.html
Learn and support:
https://helpx.adobe.com/support/stock-contributor.html
Quality:
https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
