Keyboards were designed for typing.
Gaming keypads were designed for gaming.
Gaming keyboards are a hybrid between the two, but take up far too much space when gaming.
Of course, many XIMmers game on PC as well as console. PC users need to type as well, so a gaming keyboard does the job. You don't need very many of the keys for gaming though, as one hand is on the mouse, so gaming keypads, being compact and having a small footprint, as well as usually including a more ergonomical design, are perfect for gaming. I use a keypad, with a KVM switch so that I can use it on my PS4 and my PC without having to replug it when I change from one to the other, but I have a full-sized keyboard plugged into my PC as well.