I'm pretty sure this game was created as a perk for Nintendo's subscription service.
Also, I can't play free online multiplayer games on my Xbox One without paying for Gold membership.
Oh maybe its just PS4? I know games like Fortnite, Warframe, World of Tanks can be played without PS Plus.
But ya I guess it was a perk for Nintendo subscribers since it was shown on Nintendo direct.
Sony and Nintendo allows free to play games to not require an online subscription. Microsoft requires a online sub for free to play games.
This game is considered to be a special offer to subscribers, that's why it not free to everyone. There is no forms of monetization in this game, so its understandable why it's only for subscribers.