In the case of the 30.06 (I own one), I'd suggest ultimately it's because it turned out to be a better hunting cartridge than perhaps it was as a military caliber.
Read an interesting, and quite frank...if not controversial...article some time back, in which the author related some not so popularly recalled history of the development of the 30.06.
Having had their butts somewhat kicked during the Spanish-American war by the 7mm x 57 Mauser, creation of a better rifle led to the 1903 Springfield. Hand in hand with this came the desire for a better caliber to go with it than the 30.40 Krag. The thinking of the day was "bigger is better".
Bigger than the 30.40 Krag, bigger than the .303 Brit, and definitely bigger than the 7mm Mauser.
However...
The author was critical, as apparently were many at the time, of this decision. The resulting 30.06 Springfield was brutal with regard to recoil, certainly when compared to the current standards afore mentioned. Steel butt plates coupled with this recoil made for a somewhat miserable rifle with which to train troops, many with little or no experience with heavy caliber firearms, relative to the lever actions of the day that were most prevalent.
For many, it made for generally poor marksmanship, whether on the training range, or in the field of battle. Hundreds of rounds over the course of day quickly took its toll on a soldier's nerves and/or stamina.
Logically, the 7mm Mauser was a superior caliber for the military...or arguably even the 6mm Lee Navy. That NATO, prompted by the US, ultimately adopted the much lighter and less abusive 5.56 is testament to this truth.
But the 30.06 turned out, post WW1, to be a hell of a hunting round for both N. American and African big game...most notably, including those of the dangerous variety.
An abundance of surplus...meaning cheap...military rifles and ammo following the war made the decision for hunters to upgrade from their traditional lever guns infinitely easier.
The creation of the Super .30...i.e., .300 H&H Magnum...was for the most part an attempt by the Brits to jump on the 30.06 bandwagon. The fact that it was too long for standard 30.06 actions, and that it was rimmed to accommodate British doubles, didn't help their cause, or it might have done better over the long haul.
It's somewhat ironic that what started out to be a design based upon misguided criteria predicated upon nationalistic ego as much as anything else, turned out to be such an unintended smashing success in the marketplace...forget about how many other resulting and exceedingly popular cartridges owe their pedigrees to the good ol' 30.06.
All that said...
Nothing lasts forever, eh? The days of the 30.06's popularity are numbered, albeit that horizon is still a ways off.
Just about every conversation regarding, or validating, any new midsize caliber starts off with, "It's an improvement over the 30.06 because..."
And thus, sooner or later just about everybody is switching to something better, real or imagined, than a 30.06...as a
survey of forum members here will almost certainly attest to.
