Flights to international destinations are often cheaper than many domestic flights - Why fly across the country when you can often fly around the world for around the same cost?
Don't forget that many flights to popular international destinations (London, Paris, Hong Kong etc) often sell out well in advance - however sometimes last minute flights are the way to go. The cheapest time for flights to Europe is usually around October and November where you can often fly from Vancouver to London for as low as around $500.