Why these photos got rejected?

Hello,

The first thing that you need to keep in mind is composition. Your shots are not compelling. For the rose, you really ought to choose a better background.

Composition is an important aspect in photography. Read about composition in the following link:

Composition:
https://www.adobe.com/creativecloud/photography/discover/photo-composition.html

Also read about exposure.

Exposure:
https://www.adobe.com/creativecloud/photography/discover/exposure-in-photography.html

Flower images are available by the millions on Adobe Stock's database, so they generally rejected unless they top quality or unique in presentation. In any event, this example is underexposed.