Because horizontal buttonholes take horizontal stress with less deformation of the buttonhole shape and offer much less likelihood of the button pulling out from such stress than do vertical one
It's the sign of a quality shirt. If you think of the amount of strain on the buttons, the one at your waistband and the one right at the top get the most strain. Both of them are horizontal as this puts less strain on the cotton holding the button on
From my research this is a vestige of when dress shirts were affixed to trousers. It prevented the shirt from riding up and kept a nice tidy appearance throughout the day.