A second reason is that wide-scale, reliable global satellite temperature measurement has only been possible since the 1970’s. Before that, temperature monitoring was not systematic.
Until the 1880’s, temperature recordings were mostly in either North America or Europe, and hence show major data biases. Another crucial bias was that many weather stations are in or close to cities, which grew and warmed as they burned more coal (and, later on, more oil and natural gas), causing the heat island effect. The cities, growing gently warmer, also grew toward the weather stations, usually located on the outskirts of cities, especially the stations at airports.
For example, there are two weather stations in Winnipeg – one at the wind-swept airport and the other in the heart of downtown at the Forks. An analysis back in 2007 showed the temperature difference between the two locations to be 1.57 degrees warmer at the Forks. So closing or ignoring the airport temperature measurement location would “on paper” show warming in Winnipeg. It will be the same with most major Canadian airports.