One of the main reasons (if not the main reason) why I enjoy updating Plasma is the excitement of getting a new wallpaper? How come there was no competition or wallpaper fanfare for 5.17?

That's because with the relative stability of Plasma 5 releases, they decided to switch to doing new wallpapers every other release: https://kver.wordpress.com/2019/01/17/on-wallpapers/. So Plasma 5.15 didn't have its own wallpaper, and neither did 5.17.
I don't know if they're going to do another wallpaper competition for 5.18 or if they're just going to assign someone to make it; but if they are they should probably get that started soon in order to get the new wallpaper finalized in time for that release.