Because Surface Go have a 3:2 aspect ratio screen, while iPad uses a 4:3 aspect ratio for their screen. Which means unless they increase the bezel size, the keyboard will have to be smaller.
The iPad Pro 10.5 smart keyboard cover is just 5mm wider than the Surface Go type cover. But there's a noticeable difference in the spaciousness of the keys. Here's a pic - note that the iPad Pro keyboard is closer to the camera.