Because games like Madden and Mario Party are two totally different concepts.
With a game like Madden, you're playing football. That's it. The game is the same, and the football fields are the same. There's an updated roster every year, and maybe there's some graphical improvement here and there.
With a game like Mario Party, it's a board game. Sure, there's different "rosters" of characters in Mario Party, but you're not playing the same board or the same mini-games in each title. It's the same game as the previous title, but with each title there are new boards and mini-games. It feels like a new game, so-to-speak.
I do agree though that Mario Party 8 is pretty mediocre. :\
I haven't play every single Mario Party, but I assume that some of the same mini games have been carried over to some of the newer titles?