Because (I live in South Texas) 70% of the planet has never seen snow or ice, and are pretty clueless about the various sports being showcased at winter olympics (their loss, I agree, I grew up on skis in Switzerland). Summer Olympics are much more popular. I found this online: "The 2010 Winter Games in Vancouver, Canada, were watched by 3.5 million people worldwide. The most recent summer games, held in Rio de Janeiro in 2016, drew in an audience of 3.6 billion viewers."
3.6 billion (which was the entire population of planet Earth in the early 1970s!) versus 3.5 million? A better-than-thousandfold difference? Holy smokes.