I don't quite get why my photo was rejected

You could try the following:

• sharpen the blossom without introducing artefacts
• add noise reduction to the background
• add some vibrancy
• add some local contrast (texture)

You may try to resubmit after this. But the blossom needs really to be sharp as that is where the eye will be guided to.

Setting the aperture value higher may help. I would guess that at f8 the background will still be blurred, but the rose would be somehow sharper.

And not to be ignored:

The lens you used should be crisp sharp (prime lenses normally are, and you used an L lens), but you may need to calibrate the focus, as even a slight front or back focus will cause trouble.

What was the reason Adobe Stock gave for rejecting it -- technical Issues or something else?  See links below.

• https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
• https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

Look at what other Stock contributors are doing in the same keyword category. 

• Red Roses
• Roses
• Flowers

It's not like they don't have millions & millions of roses already.  To be accepted in a fiercely competitive category like this, your work must be visually & technically perfect and special enough to set it apart from all the rest.

Hope that helps,