It wouldn't be a problem for Microsoft to build a very similar port that could deliver 120W, it would porbably be slightly bigger and require some changes to the power circuitry, but that would break backwards compatibility, which I'm assuming is the reason they stuck with the current one.
Edit; The USB-C spec allows for max 5A@20V which gives 100W, so there's no going around that if you wanna call it USB-C.