They didn't.
"Calibers available for the Contender were initially limited, stopping just short of the .308 Winchester-class rifle cartridges. However, almost any cartridge from .22 Long Rifle through .30-30 Winchester is acceptable, as long as a peak pressure of 48,000 CUP is not exceeded. This flexibility prompted a boom in the development of wildcat cartridges suitable for the Contender, such as the 7-30 Waters and .357 Herrett and the various TCU cartridges, most of which were commonly based on either the widely available .30-30 Winchester or .223 Remington cases. The largest factory caliber offered for the Contender was the .45-70, which, although a much larger case than the .308, is still feasible because of the relatively low cartridge pressures of the original black-powder round relative to the limits of the bolt face of the Contender receiver. Custom gunmakers have added to the selection, such as the J. D. Jones line of JDJ cartridges based on the .225 Winchester and .444 Marlin. Other barrel makers pushed beyond the limits the factory set, and chambered Contender barrels in lighter .308-class cartridges like the .243 Winchester. The Contender can fire .410 bore shotgun shells, either through the .45 Colt/.410 barrel or through a special 21-inch (530 mm) smoothbore shotgun barrel. A ported, rifled, .44 Magnum barrel was made available for use with shotshell cartridges in a removable-choke .44 Magnum barrel, with the choke being used to unspin the shot from the barrel rifling, or, by removing the choke, for use with standard .44 Magnum cartridges. The degree of flexibility provided by the Contender design is unique for experimenting with new cartridges, handloads, barrel lengths, and shotshells.[2]
The original Contender is now known as the generation one (G1) Contender and was replaced by the G2 Contender in 1998. The new design is dimensionally the same as the original Contender, but uses an Encore-style trigger group. Due to the changes in the trigger mechanism, and to differences in the angle of the grip relative to the boreline of the gun, the buttstocks and pistol grips are different between the G1 and G2 Contenders and will not interchange. The G2 uses essentially the same barrels and fore-ends as the original Contender and barrels will interchange, with the only two exceptions being the G2 muzzleloading barrels, which will only fit the G2 frame, and the Herrett barrels/fore-ends, which are specific for use only on a G1 frame."
