As others have said it's a matter of ammo availability, but the real driving force has more to do with the selection available. How many different factory loads are available for the .308 Win? Probably close to 100 if not more. How many different factory loads are available for the .300 Savage? There's five - one from Remington, one from Winchester, one from Hornady and two from Federal. Worse yet is that all the manufacturers only do a seasonal run of each load every year so if you don't catch it when it's available you'll be stuck waiting for them to run it again the following year. DoubleTap and a few other outfits offering production grade reloads offer a few more options, but they're typically much more expensive and are only carried by local shops and a few of the larger online retailers.