On my personal channel, I am doing some trial-and-error with different types of videos. One of my videos is marked as "not eligible for monetization" 5 days after uploading. At the moment, there is no copyright issue.
After some research, I've found a discussion on YouTube product forums. Here is the main message:
For example, videos with content that does not appear to be "family safe" or advertiser-friendly may not display ads. YouTube monetization policy:
If your video contains sexual or sensitive content you may not see any ads. We currently do not run ads on web pages that are determined to contain potentially sensitive content by our automatic contextual advertising system. On such pages, no ads may appear. We understand that not all pages automatically categorized as sensitive will contain such content, but currently, we're not able to manually alter the system to allow it to show ads. If you'd like to ensure that your video pages show ads, please keep both the video content - as well as the video title, description, and keywords - family safe
Apparently, my video contains some sensitive content. The title is "What do Muslims think about Christmas?", where I try to point out poverty in the world and food waste. YouTube thinks it is about religion and categorizes it as sensitive. I've also written a message to YouTube help. I'll let you know if there is a reply.