So a game sold at more that 15 million and which is not a system seller, that's quite an accomplishment I guess.
You need to look beyond just the lifetime total. Some people put way too much importance on lifetime sales when it comes to assessing the impact of software on hardware. Absolute lifetime numbers are less important when it comes to causing spikes in hardware sales than initial sales. Some games start off huge, amassing much if not most of their lifetime total very early on. These are the games typically associated with immediate spikes in hardware sales during their release week/month. Others start off slow but gradually amass a large lifetime total over a long time period, but don't cause any obvious growth in hardware sales. Mario Party is one of those.
Super Mario Party, the most popular to date in the series, may have sold nearly 15M copies, but it didn't start off strong. In Japan it sold only ~143k at retail in its release week, and there was no significant increase in hardware sales that week (only +6.5%/+2805 units from the previous week, indistinguishable from the normal week-to-week fluctuations). Two years later, and it is still regularly in the Top 20 and had amassed nearly 2M copies at retail, a milestone it should be passing this week. So, while it didn't cause any obvious hardware growth, it did sell very well over the long term. While we never got any leaks for its U.S. sales, it doesn't appear to have had a strong debut, placing #9 overall. The Switch's sales in its early time frame don't appear to have budged much. But it's still in the NPD Switch Top 10 and managed to just barely get back in the overall Top 20 in August.
We see the same thing from previous Mario Party games. Modest debuts not associated with any appreciable increase in hardware sales, even from more popular entries in the series. "But how could a game that popular not be a major system-seller?" Easy. It happens all the time. It's not just Mario Party. There have been more than a few games that did well lifetime but didn't cause any appreciable increases in hardware sales. They were slow-burn evergreen titles, often but not always selling well enough to stay on the charts and going on to sell well lifetime, but they had modest launches and hardware sales hardly budged when those games released. It may seem odd and counterintuitive that a game that sold many millions of copies lifetime didn't cause sales of its system to grow beyond some prior baseline, but it is something that happens. Your intuition is not a substitute for the facts. Reality is frequently counterintuitive.
Also, there's a difference between buying a system for a game and buying a system with a game. The former would be the game to make someone who has been holding off on a system to finally go out and drop several hundred dollars on the system. But not every system purchased is bought because of that reason. Some people buy a system because they just really wanted it and all it offers, not just one specific game. Maybe they got it at launch or close to launch to have it as early as possible. Maybe they waited a bit longer to save up the money, or because stock was scarce. Maybe they thought it was too expensive at first and it was a price cut that got them to take the plunge. Or maybe it was some limited edition model that was too cool to pass up even though they might have otherwise waited longer. Tell me, of the millions of Mario Party owners, how many of them went "Yep, this is what I've been waiting for. This will get me to buy the system" and went out and got a Nintendo system just to play it, and how many of them just bought the game to have something to play on the system but didn't buy the system because they were just that excited to play Mario Party? The sales data seems to indicate the former.
On a personal level, the only systems I initially bought just to play one specific game was a PS1 for FF7 and a 360 for Halo 3. Every other system I have I bought just to have them for their overall library and not for any specific game, though of course I did get games to go with them. I got Super Mario 64 to go along with my N64, Rogue Squadron to go along with my GameCube, NSBMU to go along with my Wii U, BotW to go along with my Switch, Killzone 4 to go along with my PS4, and so on. Hell, I didn't buy anything to go along with my Series X, though that's more due to scarcity and opportunity (I had been trying for months, and when GameStop finally had an in-store stock drop I jumped on it), and I have preordered Halo Infinite.