Could you help me why?

Everyone submits flowers. Stock has millions & millions of perfect flower images to sell. 

https://stock.adobe.com/search?k=flowers  67.8 MILLION

Unfortunately for you, there is a limit to how many people want flowers. The numbers do not tip in your favor.  Even if your flowers are accepted, there's no guarantee that you'll make a sale. 

To be a successful Stock photographer, find a need and fill it.  Don't do what everyone else is doing.  Be the BIG fish in a small pond.  Make lists of things that interest you and compare with available Stock inventory. 

Also read your Contributor User Guide 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
• Model/Property Releases:
  https://helpx.adobe.com/stock/contributor/help/model-release.html
  https://helpx.adobe.com/stock/contributor/help/property-release.html
  
  

Good luck.