Why did my baron steal attempt fail?

Earlier today I tried stealing baron from the enemy team. Needless to say, I failed, but I want to know why. I saved the replay from that game and uploaded it on YouTube in the hopes someone here could answer my question. To be clear, my smite damage was supposedly 760 at the time I cast it as can be seen in the video as well. Any serious comment is appreciated. https://www.youtube.com/watch?v=UHdQkn44iZg
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