There's no doubt that deer, turkey, and waterfowl are driving the hunting market these days. However, my take on "why" is a little different. I think it's about the competition, the gear, and access.
There's no record books for pheasants. Without a point system there's just not much opportunity to turn pheasant hunting into a d***-measuring contest and figure out who "won". Limits of birds are nice, but nobody mounts a limit so you can't show it off. At the taxidermist's booths at sports shows, how many upland birds do you see?
Also, the gear requirements have traditionally been much more modest: a shotgun, a decent pair of jeans, a coat or a vest, an orange hat, a pair of boots, maybe a dog. Go look at the Cabela's in Wichita, KC, Omaha, even Kearney and Sydney and see how much floor space is dedicated to upland. Some might say that these things are a result of the popularity, not the cause of it, but I believe it's self-reinforcing. That's particularly true when you consider the advertising that masquerades as "programming" on the hunting and fishing related tv shows. Marketing dollars are spent on growing markets, not just taking market share from competitors.
Finally, and the importance of this point can't be underestimated: you only need one place to provide a season's worth of hunting access for each of these other species. In some parts of the state or country, that place could be really small. What's a bowhunter need? 40 acres maybe? At one time I was looking at a duck hunting spot that was 40 acres, and only about 2-3 of it was water. What's a pheasant hunter need? If it were managed exclusively for bird hunting, somewhere between a half section and a section would probably produce more than enough birds for most of us. However, we would all probably get bored hunting the same section week after week.
Way off topic from the original post, but commercialization of big game hunting and more recently duck hunting, but not upland hunting, has always interested me.