As of 2017, there are 52 countries that are exempt from having to obtain visa for visits to Canada. The countries that are eligible for visa free travel to Canada and for an eTA are developed, stable, high income countries that do not pose any threat to Canada.
Illegal immigrants and Asylum claimants
In order for a country to be visa exempt to Canada and thus eligible for an eTA, the percentage of travelers from that country who overstay their six month maximum stay period in Canada must be very small. Likewise, the number of asylum claimants from visa exempt countries must be low otherwise Canadian authorities could re-introduce visa requirements.
Law enforcement and Security
All countries that are eligible to enter Canada without a visa and thus eligible for an eTA must have solid law enforcement measures. Countries that are allowed visa free access to Canada must actively cooperate with Canadian authorities in areas of security and anti-terrorism collaboration.