It seems after 40 votes, most people are thinking it is because it appeals to the expanded audience, leading with 50% of the vote. I expected that would get the most votes, but was surprised the "the Wii Mote suits the game" option, has gotten so few votes. Even less than because the "there wasn't much else to buy" choice. Do people really buy a game they don't really want just because there is nothing else?

Expanded audience.

It is a combination of 2, 4, & 5.

The Wii remote works well with the type of game, it has a larger expanded audience that includes whole families playing together, and it is the only Mario Party on the Wii. Even if Mario Party 9 was released on schedule Mario party 8 still would have beaten all the previous games sales. It was at 5m sales at week 52.

I agree with norlead, all those factors have played a part. But also like he pointed out, it would have a huge increase even if another entry launched a year later. So I think it is between 2 & 4 is the main reason for the increase. Personally I would go with 2, the Wii Remote is seemingly a perfect fit for the franchise, although I do feel it was a little under utilized in Mario Party 8.

There are more people with Wiis than people with gamecubes that fact alone can sell a game like this

I think what's really interesting is if you look at the thread the topic creator links too, that Mario Party has actually increased percentage wise more than Mario Kart Wii over Mario Kart Double Dash. That's very impressive in my opinion.

expanded audience, few high quality games, and motion controls all contributed.

It was a crap game compared to all other Mario Parties. It really pissed me off.

But it sold well because it was available when the wii came out, or shortly after. Didn't have a lot of competition, and we all know everyone had a WII

It's because of the last three options. Actually, the game wasn't very well made and the controls aren't better than average. I bought it because there wasn't much to buy at the time. My brother liked it (expanded audience) but I ended up selling it because I thought it was a terrible game.

Expanded audience