Because it wouldn't have enough power. Todds workshop has a video arrows vs armor medieval mythbusting, and they're shooting 160 pound bows into regular plate from oh, like five feet if I recall correctly. And it barely dent's it, that's in the ballpark for what your average mistborn will weigh. In normal plate, it doesn't get through. No way in the world it cracks shardplate. And if you think that Plate isn't designed to redirect projectiles you haven't looked into it very much, for starters it's rounded and the coin wll want to roll along that, and additionally the Radiant can stumble back which also causes a blow to lose effectiveness.
We have no idea how many vials of metal Vin went through
Kaladin is 2nd Ideal
Doesn't get you a kill