Unsure what is wrong with my rejected photos

It is RGB color but mostly desaturated.

Underexposed, missing highlights.

Also the camera angle is off kilter.

As a customer, I'm not sure what I could use this for commercially.

the size of 8mb is not exactly a quality indicator. It's kind of saying that a heavy car is better than a lighter car.  Unmodified is not a quality guaranty, neither. The asset clearly requires some geometric corrections. Indeed all good pictures from the great masters are modified. You wouldn't buy a car without paint, just to stick to the car analogy.

I will need to load the picture to Photoshop for an in depth analyses. But it is surely not a monochrome image, as @Jill_C suggested, even that the colour is not very prominent. So that can't be the refusal reason, IMHO.

Just some guesses. I like this piece as being an interesting composition artistically speaking, but it may be lacking in commercial value. Also, while I realize it's a color photo, it comes across as black and white and B&W is rarely accepted by the moderators. There's some clean up that could have been done on the fence door (the white specks) and it could use some sharpening. Again, just guesses. A sharpened example is attached, assuming it doesn't lose detail during the upload.

Monochrome conversions are not accepted by Adobe Stock.