John Beck told me years ago that they had made a decision to exclude many mines from the COPD list on several grounds (I'm paraphrasing, but it's roughly correct):
Practicality and time - the book would have been three times larger and three times later if it had included most of them
Safety - many of the mines just could not be recommended for entry due to the uncertain conditions
Legality - many mines have more than a landowner issue to overcome (mineral rights, legal requirement to keep entrances sealed etc.) and so recommending certain mines for visits may have created more problems. The recent trend of publicising unauthorised mine exploration on the internet, and particularly social media, may exacerbate this issue in certain areas.
Possibly also as many mines are explored by folks who don't have much interest in natural caves, and there are several mine-specific publications already out there with the space to document them more fully - like the PDMHS Bulletins ( a treasure trove of info dating back to the late 1950s).
Personally, I haven't ever been unable to find or explore a mine because it wasn't in COPD. As Cave Registry Secretary I'm always happy to add mines to the database, as this is an online resource, and so can be as large as we like - and we can include more up-to-date info as access or safety situations change. How that's filtered down into the next book remains to be seen but I'm sure there are ways to overcome the size issue. But first we have to sell the remaining copies of the current edition!