There was an increase in 2-income families, which meant more disposable income. More disposable income means a bigger budget for leisurely travel.
World War II brought about the breakthrough of the jet engine, which resulted in dramatic improvements in travel.
The concept of paid holidays and vacation became more expansive, so people had more time to travel without the consquence of missing work days.
During the war, petrol and food were rationed, making travel difficult. Rationing was phased out after the war, which boosted tourism.
The biggest reason - increased car ownership. Many more families now owned cars, making travel much easier and more likely. This increased international travel in Europe greatly.
