Under normal circumstances it's because of price and availability compared to .22LR ammo. These days that's a moot point. The shop I was in on Saturday only had .17 HMR on the rimfire ammo shelf.
How badly does a .17HMR tear up a squirrel with a chest shot? If meat damage is excessive on a small animal, I would opt to just stick with a .22RF no matter how much better a .17HMR is in terms of killing power and flatter trajectory. Head shots are great, and to hear it everywhere on the internet that's all everybody does. In the real world that ain't so, me included.
That said, I have a small Martini action that I've been intrigued about building a .17HMR on.
