Personally, I think it correlates to property and IP. Smash Bros is huge and it's a huge celebration of the best of the best of Nintendo's long and storied histories, producing merchandise for cult games with cult followings like Earthbound and F-Zero that isn't exactly swimming in merchandise yet whose fans would readily gobble it up.
... I'm not sure Kirby or Chibi-Robo or Animal Crossing have that same metric, especially as stand-alone lines.
At the same time, if Nintendo announced a line of Metroid figures - of Ridley, of Light Suit Samus, of Dark Samus, of Kraid, etc - I can see starving Metroid fans scoop them up (surprised they didn't for Metroid Prime: Federation Force...).
Zelda will still probably do well, but imagine tying it into a Hyrule Warriors line - Impa, King Daphnes, Zant, etc. - and it would have gotten passionate Zelda fans into a frenzy.
Fads fade with time, but it's surreal to me that Nintendo hunkered down and churned out nearly two dozen Animal Crossing amiibos long before they even finished up their Mario cast.