It's you doing it wrong. I insta kill quite often with my bow. All you need is assassin's gear that boost the damage of them and maybe even a ring and a necklace in addition to gauntlets. That will kill with one hit. If it doesn't, they still haven't seen you even if they start running towards you, so just shoot another sneak attack and then they definitely should be dead.
As you level up, you'll find more and more NPCs that don't die with one arrow. Want an example?
Second time I played through, I started the MQ at level 50. By then I had 100/100 in sneaking and archery as well as all the useful perks to make my character a true, unstoppable sniper. I was doing the diplomacy quest. At a certain point, there's a mage guarding a door. He's relaxed, leaning against it. No armor, no shield spell. My equipment, while not enchanted or upgraded via smithing, is impressive: I had a daedric bow, ebony arrows, a ring that grants me +25% archery damage and a cowl which further enhances it by 15%.
I shot at arrow at him from behind a bush, unseen. His health goes down only by 50%. By the time I hit him with a second arrow he put up a barrier spell and he has spotted me. There goes the bonus damage. It took me two more arrows to put him down for good while he also had time to heal himself with a restoration spell and hurl a few fireballs at me.
This is totally inexcusable. First, thanks to level scaling, enemies become HP sponges at high level. Second a 3x damage is not enough. Had I the chance to go behind him, I could have killed him outright with a sword or a dagger. A bloody stupid mage with no armor or costant effect shield spells.