Why are photos not accepted?

This image is low resolution. In no way it is what you did upload to stock.

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

Before you submit assets, compare them with current Stock inventory in the same keywords.  It should be as good or better than what Stock has now. 

As a customer, would I buy this? 

What would I use it for commercially? 

How could it be better or different from what's already available?

Adobe Stock customers expect the highest visual and technical quality for use in commercial projects. Read these links for more tips:

• https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
• https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html
• https://helpx.adobe.com/stock/contributor/user-guide.html/stock/contributor/help/photography-illustrations.ug.html
• https://helpx.adobe.com/stock/how-to/tips-stock-image-acceptance.html

Good luck.

Soft focus and low contrast would be my best guess as to why this was rejected.

What was the reason given for rejection? And why the square crop? You aren't leaving room for a potential buyer to crop it themselves or add copy if they wish to do so.