It is because of technical limitations.
It may not seems like it, but Terraria 1.3+ requires a good bunch of RAM, but the Wii U only has 1 Gb of RAM used for games (and its RAM is really slow).
That's not a matter of money, they had to drop the support for PS3, PS Vita, and Xbox 360 too, because of the same problems.
To compare it, you can check the Switch version (which is more powerful than a Wii U, but not by much), which has some performances issues maintaining a good framerate and stability in game, and it runs 1.3.5.
Unfortunately, that's how it is. Old consoles are just not up to the task for today's games.