One, they wanted to offer a new product.
Two, for a device to work in clamshell form like a laptop, the base has to be large enough and heavy enough to counterbalance the weight of the screen half. This is why the Surface Book has the odd hinge design, and why the Surface Book is lighter than the SP4 despite being a much larger screen. Getting that balance right for something as massive as a Surface Pro would mean a very large, very heavy base, needing a very beefy hinge in order to not wobble and not wear out.
Two, for a device to work in clamshell form like a laptop, the base has to be large enough and heavy enough to counterbalance the weight of the screen half. This is why the Surface Book has the odd hinge design, and why the Surface Book is lighter than the SP4 despite being a much larger screen. Getting that balance right for something as massive as a Surface Pro would mean a very large, very heavy base, needing a very beefy hinge in order to not wobble and not wear out.
