I have to imagine turning Mario Kart and Super Smash Bros. into annual franchises would eventually cause franchise burnout among the casual Nintendo consumer. I would imagine the risk of consumer burnout exacerbates the cost of having to perpetually devote resources each year to those titles.

Furthermore, since both of those franchises only come out once per console cycle, Nintendo can maintain higher prices on their games for a longer period of time. Pokémon games tend to retain value and still sell well because Gamefreak puts different difficult-to-obtain Pokémon in each installment (with ORAS being the major exception, as it has a ton of legendaries), which encourages the purchase of older titles in the series.