Originally Posted by bankops

Because a) Canada and the US decided a very long time ago that IATA codes would be their ICAO code, minus the first letter. b) You do not want to know how many global reservation systems, corporate booking engines, air traffic control, flight management software, etc, etc would have to be changed. c) other countries do not play along because they have 10% as many airports as the US and Canada so they do not need a logic system.

US 19,700 airports
US 2500 ICAO airports
US 2100 IATA airport
US 500 commercial service

Canada 400 ICAO airports
Canada 360 IATA airports
Canada 40 commercial service