I think the deal with Mario Party is Nintendo figured they had oversaturated the property. Obviously they were releasing one a year and it was easy to see how slowly the sales of the game were progressively worse. For instance here is all of the sales:
Mario Party (1st on N64)- 2.70 million
Mario Party 2- 2.50 million
Mario Party 3- 1.91 million
Mario Party 4 (first on GC)- 2.47 million
Mario Party 5- 2.08 million
Mario Party 6- 1.63 million
Mario Party 7- 1.53 million
Mario Party 8 (first on Wii)- 7.06 million
Mario Party Advance- 0.28 million
Mario Party DS- 6.53 million
Trend wise up until Wii and DS, the titles had usual sales for a once a year game. First one on a system would sell the best and then decrease in sales along the way, especially considering userbase was never growing that fast for N64 or GC. However, Wii and DS first versions released to bigger success, mainly due to being a premiere title when the Wii version released and the DS version feeding off of it later. They hit bigger sales and Nintendo opted against releasing another one the next year.
Why would they do that? Well it's the same approach they take with the Mario Kart games. Mario Kart games are long-term sellers and usually system-movers. If they were to release one very often, it not only saturates sales of the older title but it saturates the viability of the game itself. There are certain genres out there such as racers and party games, that just don't have the ability to change as much as other genres. Release less titles and keep the "freshness" higher.
I think this is something Nintendo is attempting with Mario Party franchise. So far the DS and Wii versions saw huge success without an infringing successor behind them and were able to sell over a long period of time. Might not see another Mario Party game until the next systems or later in the Wii/DS cycle.