Why would Uzzah be struck down for merely trying to steady the ark in 2 Samuel?

Because the story of Uzzah being struck down for touching the ark is making the religio-cultural point that the author wanted it to make.

It's like asking why did Icarus not listen to his father's advice to not fly too close to the sun when escaping Crete - because the author of the story wanted to make a point.

What I'm trying to say is that the story isn't necessarily a factual one. People don't die because they touched a box that is being carried by bulls away from a town where people were struck with haemherroids for not worshipping the God of that box.