Some types of domain extensions have restrictions for who can register them, requiring a local presence (address), or citizenship, or proof of professional affiliation requirements.
Where possible we try to offer these extensions, but for some of them the restrictions are so extensive that few interested parties would be able to meet them.
Sometimes the paperwork required to provide the necessary documentation is just too cumbersome, manual, or slow to be worth it for our customers.
Sometimes a registry will launch a TLD where the majority of domains are categorized as premium, and/or the registration price is extremely high. Or it's possible you are looking at a TLD that hasn't publicly launched yet so can't be registered, but only pre-ordered.
Beyond that, generally for the extensions we don't carry, demand is very low, so the level of bureaucracy required to register them just isn't worth it for how many of those domains would be registered as a result.
If you have questions about a specific extension, please let us know.