Yep, because we weren't stupid enough to put a massive derp-cannon on a medium tank chassis and expect it to work properly. Maybe that's where we went wrong. (T44-122 anyone?!)
Because Gaijin doesn't want anyone beating up glorious stronk soviet tanks too easily?
If anything, the US 76mm could be *more* effective, because when you do get a penetration, you've a decent explosive charge which may wreck the enemy tank. If you get a penetration with the 17pdr (RNG decides to allow your shell through) then you've got either the disappointment of a 'hit' only, or a critical which actually doesn't prevent the enemy from firing his shell at you, often killing or crippling you before you can reload.
At 4.0, it's a problem, at 5.0 and 5.3 the same gun is just not competitive, given the way the ammo performs.