No idea, but after the party machine that was the Wii, the lack of strong multiplayer titles on the Wii U is one of my many disappointments with the system. And I know I shouldn't judge games I haven't played, but from what I've seen, Mario Party 10 and Mario Tennis Ultra Smash looked kind of bare-bones compared to past games in the series, and not very worthwhile. I still might check them out if the price is right, but I go back and forth on that.
In the case of Mario Party, somewhere around 2009 it switched hands from Hudson to Nd Cube, and although the company retains some of the Hudson staff, they still work on a different schedule and make different decisions. One thing that happened is that they made Wii Party and Wii Party U, which is similar to Mario Party with Miis instead of Mario, but has some differences. So that has lessened the frequency of Mario Party titles.
Also, the best-selling Mario Party game is Mario Party DS, so it makes some sense they would make 3DS versions when the Wii U has such low sales potential for games.
I do suspect that the budget of HD games means these kinds of B-tier stuff can longer be very feature-full, and apparently sometimes not even as feature-full as past games. It's why we're seeing all the spin-offs appear on 3DS instead of Wii U, where they're far cheaper to make. A real shame, as it feels like graphics has won the war of gameplay vs graphics.