LogFAQs > #968093407

This should give a clue:
From: https://en.wikipedia.org/wiki/Star_Fox_64#Development
"By 1995, Miyamoto and team had largely finished development of Star Fox 2 before realizing that its release would require the upgraded Super FX 2 chip, which would increase the cost of the game.[16] Furthermore, newly launched competitor consoles, namely the Sega Saturn and the Sony PlayStation, possessed processing power and graphics capabilities that dwarfed those of the SNES, making Star Fox 2 appear obsolete.[17] Recognizing this and that the Nintendo 64 would launch the following year with significantly increased processing power and graphical capabilities, Miyamoto made the decision to cancel Star Fox 2 in favor of developing a new Star Fox game for the Nintendo 64."

The Donkey Kong Country series didn't require an add-on chip as elaborate and expensive as the Super FX 2 chip. Additionally, DKC being pre-rendered 3D had it still looking decent compared to next-gen offerings. The Super FX 2 chip couldn't get anywhere close to the next-gen system in real-time 3D capabilities. Star Fox 2 barely even had textured polygons.