ccTLDs are the two-letter TLDs assigned to individual countries based on their ISO country code, like .us, .uk or .de. All two-letter TLDs are ccTLDs, even though some (like .me and .ai) are marketed otherwise.
We are very hesitant to allow adding ccTLDs for countries other than the United States. Doing so raises concerns about giving those countries political, economic, or legal leverage over us or our members. Most ccTLD operators are part of or run on behalf of the country's government. They typically include things in their registrar agreement like "Paragraph 1219: You will follow all of our country's laws." and "Paragraph 2751: The ccTLD operator reserves the right to terminate this agreement at any time for any reason." That's a problem.
For example, suppose that we offered registrations in Atlantis's ccTLD and accumulated a few thousand domains. And maybe the Atlantis government decides they don't like a site we host that criticizes their land subsidence policies. Next thing we know, they're threatening to seize all those domains unless we cut somebody off, and claiming that we agreed to follow their laws on such matters.
While that may sound farfetched, we have had conflicts with foreign governments over member sites, and they don't play nice. Handing significant leverage to people who may not have our members' best interests at heart doesn't seem like a good idea.
Many ccTLD's also have weird, special requirements and procedures that would require a lot of extra work for us to support. (Looking right at you, .uk, and your IPS tags!) That may not sound like a big deal, but "extra work" for us translates directly to "extra cost" for you. Some ccTLDs even have complex legal requirements that the registrar, the name servers, or the registrant be physically present in that country that would be difficult or impossible for us to meet, even if all the other issues didn't exist.