Why does Kalista suck so much at taking down turrets?

I noticed this in my last game. We were trying to take down the enemy midlane turret after acing the enemy team and had no minions. We were there with 2 people, me and a WW, tanking the turret, but couldn't destroy it. At that point I had {{item:3153}} {{item:3085}} {{item:3139}} (+ {{item:3065}}, because the enemy AP {{champion:54}} still kept oneshotting me even with Mercurial), which means that I didn't have a lot of AD, but with {{item:3046}} {{item:3153}} {{item:3031}} it is only 5% attackspeed and 5 AD more. Is it because of {{champion:429}}'s passive "Martial Poise"? > Additionally, her basic attacks only deal 90% AD physical damage. I mean, that does make a difference, but it still seems odd, that it renders her worse at destroying turrets than your avarage AP mid with Lich Bane. Seing stuff like this https://www.youtube.com/watch?v=uS5xI8jARQs&feature=youtu.be&t=15s while trying to get good at Kalista is really sad.
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