Well ages ago the common handgun caliber was the .44. It started out being used in cap and ball revolvers and then cartridges came out so they just kept the same caliber. At the time it was more popular than the .45 Colt too. So a .44 Magnum makes sense today as you can shoot .44 special or 44 mag loads in it. Maybe the venerable .38's would have been the most popular, as way back then it was at first used in the .36 Cap and Ball revolvers and then later used in cartridges too. Many people bought and used them in the past and still do so today too. Thus a .357 would use Magnum or .38 Special loads.