[2019] The upgraded builds from STW should be available in Creative.

So the reason is only technical. The higher tier structure variants actually take up a lot of valuable memory on all platforms. Remember the many various combinations you can edit a wall, floor, ramp or roof into? Each combination takes up it's own memory, and player-built structures are omnipresent in Fortnite -- they always need to be loaded and ready to go, because they can burst into existence at any moment.

That said, of course we want to make these available, and as we continue to improve Creative we should be able to.