From their official Blog
Stardew Valley will not be coming to the Wii U, however. Looking at Wii U and the current gaming landscape, we made the difficult choice to shift our development efforts to creating a version for Nintendo Switch. I am really looking forward to the Switch, and I believe that the Switch’s unique capabilities could create some exciting opportunities for Stardew Valley. I do want to emphasize that I am absolutely committed to seeing Stardew Valley on Nintendo platforms, and I really do appreciate your support and understanding in this.