What's wrong in it , why it got rejected

I think they're over-simplified compared to Adobe's new standards for quality. They lack some line work and basic shadows.

You'll need to be more innovative if you expect to compete with current Stock inventory.  As a customer, which would you buy?

https://stock.adobe.com/search/images?k=christmas+tree+vector 2.8 million results

https://stock.adobe.com/search/images?k=christmas+ornament+vector 6.4 million results

Adobe customers are looking for commercial quality graphics to use in professional projects— print, digital & textile. Also, most vector graphics are offered as collection sets.  This gives customers more options for use in multiple projects.

Finally, submit vector graphics as Adobe Illustrator .ai, .eps or .svg files.  Math-based vector graphics retain their crisp quality when rescaled. You can't do that with pixel-based JPEG or transparent PNG files.

• https://helpx.adobe.com/stock/contributor/help/vector-requirements.html

Hope that helps.

PNG file type is supposed to be used only for images with transparent background.