The reasons they're rare are not because either cartridge is bad or that barrels are hard to get. Any time you like you can order a custom shop barrel for either cartridge. The fact is, not than many people want guns chambered for either cartridge.
In the case of the 17 Remington, not that many people want them since barrels foul more easily and are harder to clean than larger diameter barrels, ammo is harder to come by than that for the 204 Ruger, 223 Remington, 22-250, etc. and the windbucking ability is less than the .224s.
The 300 H&H Magnum is, face it, obsolete. Ammo is not in as good supply as say 300 Win. Mag., case life is poor because of the design, etc. It's a classic cartridge and a good cartridge but not as good when all things are considered as a 300 Win. Mag. or a 300 Weatherby Mag. I, by the way, shoot an Encore in 300 Win. Mag. and the recoil isn't that bad and it wouldn't be any worse in a 300 H&H Mag.