The Tiger was only really vulnerable to either 76mm or 75mm up close from the rear, and even then it wasn’t a sure thing. Panthers were a different game, as their sides were much thinner, even if front-on they were too thick to penetrate. The 17pdr was a beast of a gun, with high pressure and thus muzzle velocity - the US almost took some conversions, but as the war was almost over they decided not to complicate supply lines etc.
We need to bear in mind that Tigers were meant to be a breakthrough tank, surrounded by thick armour so they could slog their way straight through enemy lines. The protection meant lots of weight, so they were complicated and expensive to build. Shermans by comparison were general workhorses, so weren’t optimised for any one task.