Why might a domain not be available for registration?

Question

Domain name market

Why are some domains not available for registration?
On some service they write about an incorrect domain name, on another they write that it is busy, on the third they write that the domain is not available for registration.
On the example of **.gr - what's in it? There are similar hh.ru, i.e. it's not about the number of characters - what then?

Answer 1

Different domain zones have different restrictions. Domain names in the gr zone must have a minimum of 3 and a maximum of 60 characters. At least one of the registrars assures of this. The site of the main coordinating organization of the GR zone says that there can be two-character . But this information is from 2015.
There may be a subjective restriction, such as two-character only for members of the trade union.

Answer 2

1-3 characters can be sold for good money
Only the sale of such domain names can pay off all the gestures of registering your domain zone
Consider that this is such a legalized cybersquatting