About image rejection

There's too much competition from other background & wallpaper images -- free as well as paid. Besides, anyone with decent Photoshop skills can create their own backgrounds.

If you're looking for inspiration, see below. 

https://helpx.adobe.com/stock/contributor/help/artist-hub-migration/creat-what-s-in-demand.html

Stay away from overly represented content like backgrounds, clouds, sunsets, flowers, plants, trees, leaves and pets. Stock already contains more of these than they need.

Focus on real life situations at home, school or office. The more diversity you can bring, the better.  Aim for commercial quality images that will be suitable for use in print, digital and on merchandise (t-shirts, mugs, tote bags).

Hope that helps.

Regardless of the reason for rejection, abstracts the likes of the one you posted are available for free on the internet or can easily be created by a graphic designer even with little or no AI experience. Unless the backgrounds are extremely unique, they are best to avoid submitting.

Your image file is too small, so it is not the asset as submitted. There is no possibility to check the quality, but as this is an abstract, it may anyhow be difficult to understand the rational behind the rejection.