To expand on my comment about Smash not having the legs of Mario Kart, here's comparing the legs for the Wii, 3DS and WiiU entries in these series. Lower First Year % means stronger legs.
||Sales 52 Weeks from Launch
||First Year (%)
|Smash Bros Brawl
|Mario Kart Wii
|Smash Bros 3DS
|Mario Kart 7
|Smash Bros WiiU
|Mario Kart 8
These are retail only sales, as you can see Mario Kart typically has the majority of it's sales outside it's first year of release, whereas Smash Bros has the majority of sales within it's first 52 weeks on sale. This is because Mario Kart has a much wider appeal towards the more casual gamer, as such it sells to new people picking up the system for a long time. Whereas the majority of people interested in Smash Bros lean more to the hardcore, and are very likely to already own the system, or purchase the system specifically for Smash relatively soon after it's release
The WiiU releases are a lot closer when it comes to legs, though this can be explained by the WiiU being brutally murdered.
Taking an average from these 3 games in each series, Smash Bros has 72.1% of it's lifetime sales in it's first 52 weeks, while Mario Kart has only 49.4%.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)