I thought this over, and there could be two possible reasons for why we haven't seen another Mario Party yet:
1. Hudson just doesn't want to do it, plain and simple.
2. Nintendo wants to make a Wi-Fi edition for either Wii or DS/i, but they might be running into some problems.
Or the non-casual/casual audience just don't care about Mario Party anymore. MP DS is a good local multiplayer game (cardless mini-game marathon is the ****), but having no WiFi kind of killed it some.
Also, Wii Sports is a better party game.