"Why a one-sider ?" is for the same reason as any other box that goes hunting with me.
SOUND
Other than authenticity in sound, there's little other reason I carry a call around the woods. I don't discriminate based on the number of rails.
The first box call I bought, some 40-ish years ago, was a one-sider. But, sentimentality be darned. It's ALL about the sound.