I think that the 30-06 is about the biggest cartridge the "average" shooter can handle in an off-the-shelf rifle of standard weight, and it will handily knock down anything he's likely to shoot with it on this continent. The long history as a military round ( just like the 308 ) is another thing it has going for it, and the availability of military surplus ammo used to be a big selling point. The guy who only shoots two boxes of cartridges per year can afford to feed it and still have enough cash for a case of beer to take to camp that weekend. There's a ton of casual shooters/hunters out there, and it works well - even with ordinary bullets.
I grew up in the era of the one-rifle hunter, and darn near everybody was shooting 30-caliber in those days. The story was that if you ever go west to hunt elk, or north for moose, the '06 is big enough - and it will certainly kill our skinny little whitetails handily enough. I think that everybody understood that it was way more horsepower than needed for deer, but there was always that "all-around rifle" concept being discussed around the hunting camp card table. I don't know anybody who didn't dream of hunting bigger animals in some far-away place some day, and this is what I think fueled the popularity of these cartridges. Having Grampa's 30-06 handed down to Junior didn't hurt its popularity any, either.