This question hardly has anything to do with Expats but I'll toss out an opinion before I vote to close.
The United States has a far larger GDP, range of climates, variety of sub-cultures, many levels of education and more numerous, and many many more population centers. All that translates to far more opportunity to live the kind of life that many in the Western world would like to live. Nothing's guaranteed ofcourse. There aren't as many safety nets in the U.S. as in many European countries (or Canada) for example, but the potential quality of life, especially for those who are hard-working, a bit skilled, and a bit clever, is far higher than anywhere else in the world. Further, realistically speaking, this looks to be the case for many decades to come.
Cananda is a lovely country. They have strong trading/culture partner and a powerful military ally right next door with whom relations are historically superb. But it is a widely dispersed population and rather homogenous as far as opportunity is concerned.
Plus Canada just hasn't had the cultural reach and impact that the United States has had. When people think of, even considering only as a wild possibility, the idea of attempting something like birth tourism, I'm not sure how many of them would even briefly acknowledge Canada much less realize that such a thing was possible with that country. You'd have to have been living under a rock for the last seventy years to not know this was possible with the U.S. (due to Hollywood and word of mouth).