1. Information where improper disclosure is illegal. For example, health records. Or credit card details, which would presumably be a violation of PCI DSS.
2. Information that can be actively exploited, but can also be fixed so the previous disclosure is harmless. This means passwords, authentication tokens, etc.
3. Information that is merely private in nature.
Cloudflare is focusing on the first two items. The third one is hard to quantify; what one person may consider private, another person might not care about. And there's not much that can be done about this kind of disclosure (beyond scrubbing caches which they're doing anyway). Also, it's difficult to automatically identify this type of content (whereas cookies, passwords, credit card numbers, etc are pretty easy to detect) and Cloudflare probably doesn't want to have their employees spending their time reading through all of the cached data they can find looking to see if there's private info (both because it's a lot of work and a huge waste of time since it won't affect anything, and because it's private info; chances are nobody's going to see it normally, and having employees reading your private messages doesn't help anyone and is a violation of your privacy).