The bottom buttonhole, along with the top button at the collar, are usually sewn horizontally to allow the button to take more stress from pulling and movement without stretching out the shirt or the hole itself. It's also why they can often have reinforced stitching with thicker thread. Traditionally, this was done with a thread of a different color. The thread color has stuck around more as a stylistic choice. But the horizontal buttonholes remain imperative to reduce the chance of a popped button.