Their participation rules from 2017 explicitly exclude residents of certain countries:
The Contest is open to individuals who are [...] (2) not a resident of Brazil, Quebec, Cuba, Iran, Syria, North Korea, Sudan, or Crimea;
Due to US embargoes, it's plausible that Cuba, Iran, Syria, North Korea, Sudan and Crimea are excluded from the competition (which also features a monetary reward). But why are Brazil and Quebec on the list, too? The FAQ only makes a vague reference to legal difficulties:
Why are Brazil and Quebec excluded from the CTF?
Local laws and regulations make it extremely challenging for us to run a competition open for residents of those locations. We are truly sorry about this, and hope to change this in the future if possible.
Could you give some insights into what particular legal challenges might have led to the decision to exclude Brazil and Quebec from the CTF?