Why? Part of the absurdity of the trial is that this is what they thought should be the nail in his coffin. Even though Trump did things far worse than banging an unsavoury broad and paying her off. Myself I couldn't care less about the sexual relationships of a candidate unless they're security-relevant. (Granted it is absurd how Americans went from Gary Hart to Donald Trump in just over thirty years.)
In Trump's case, I wouldn't even care about him paying that bint off because I'm sure their affair becoming public knowledge would not have had an effect on his chances. "Grab 'em by the pussy" didn't harm his chances, not even in the Bible Belt; so why would Stormy-something be a cause for concern? He probably just wanted to salvage his marriage, or maybe his team believed they had to protect him.
But Trump's success is due to him not giving a toss about anything. The media would accuse him of some sort of wrongdoing, and where all the other politicians would be quick to grovel and apologise, Trump would simply put up a big stupid grin and be like "of course I did that". That's why he's so untouchable and surviving scandal after scandal that would've ruined every other career.