Most of my photos are being rejected — could you help me understand why?

Before you submit content to Stock, read your Contributor User Guide. 

Start with the 4-part photography requirements below.

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-submission-overview.html

Also read about Moderation, Known Image Restrictions, Quality & Technical Issues. In some cases you’ll need signed Model or Property Releases to submit your photos for commercial use.

https://helpx.adobe.com/stock/contributor/content-policies-guidelines/model-property-releases/property-release-overview.html

Good luck.

Little (or no) commercial value. The highlights and shadows need to be fixed. The best image is the two towers (if lighting is being fixed) - but the modern roof in the foreground ruins the image. Why not go to the other side of the towers?

Always check your histogram.

Exposure and some cropping errors

Images with blown-out highlights or crushed blacks will be rejected.
Please shoot in RAW format and process the images in Lightroom before submitting them.

Let's start here.
https://helpx.adobe.com/stock/contributor.html

Hello,

Composition could be better. For the church steeples, you have too much light from the sky. The background is too bright. Avoid taking photos against a strong light background.

With this shot:

the tree trunk is in the way!

So, there are problems with lighting, composition, subject choice, commercial value...

I think the main problem is indeed with the quality and suitability with the photos.

Remember that the photos being used are for commercial purposes, so you have to think how they can be used. What is the commercial value in these shots?

Read about composition from Adobe:

Composition