Why did they? Why not? I suspect old Charlie Newton bent someone's ear until they gave in and chambered a few.
What's wrong with killing stuff with a .22 centerfire? The .22 HP is no better or worse than the .223, and a lot of folks are going around killing all manner of deer sized game with that one. May or may not have been as valid a point 100 years ago due to advances in bullet technology, but still, HP's were always loaded with tough bullets. It was always considered to be a medium big game cartridge not a varmint round. Stick a deer in the guts with a .30-06 and it'll run a helluva ways before going to ground. Stick a deer in the guts with a .22HP and the same thing happens. Stick that same deer in the heart or spinal column and you'll get the same results whether it be with an '06 or a HP.
Advantages of the .22HP: Low recoil, accurate, deadly, found in delightfully handy woods rifles.
Disadvantages:
Bless those Savage thinkers for they gave us loonies a bunch of neat cartridges.
