The reason would be "in the editor's judgment, it would have been a violation of the editors' guidelines to list the sites." The editors' guidelines are, of course, public.
You might review your sites with this in mind:
1. Are they related IN ANY WAY, such as being based on the knowledge of the same person, or being commerce sites offering services of the same entity, or being advertising sites operating to the financial benefit of the same entity, or in any other way? If so, submitting both would be a violation of the submittal policies, and according to the submittal policies, removal of both is the prescribed action.
2. Do the sites primarily exist to promote the commercial offerings of some other entity -- which also offers goods or services via other websites? If so, the "duplicate content" rule is obviously involved.
3. Is there a SIGNIFICANT amount of authoritative information available on these sites and not available elsewhere on the web? is it easy to find on the website? is it easy to distinguish from other content (such as advertising or non-unique information) on the site? If the answer is not "yes" across the board here, the unique content rule very likely applies.
You know what (if anything) you know, that nobody else knows, that you published on your website. There's really nothing we can EVER tell you about that, because YOU KNOW that, and WE generally do NOT know everything about that. And the explanation basically always lies in what you already know.
You're welcome to ask your friends to review the ODP guidelines and your website, with "unique relevant information" in mind, to see how an editor would have approached the site. But it's basically never a good idea for an editor to get between a demanding webmaster and the guidelines. (The editor is responsible solely to the community and the guidelines.)
And you are free to re-suggest one of the sites: it is, however, in my experience, vanishingly unlikely that it would be re-listed: and my recommendation is always to spend that three years not "waiting" but "doing what you'd do if you expected the it not to be relisted."
You're free to re-suggest both sites, if you want to make sure that the reason they aren't listed is the submittal policy. But I do not recommend that approach.