Donkey Kong got his original Game Boy spin-off because it was too hard to port Donkey Kong Country

gamesradar.com:

Donkey Kong Land owes its existence to Battletoads and the challenges of trying to port Donkey Kong Country to Game Boy.

Rare cofounder Tim Stamper approached programmer Paul Machacek about creating a Game Boy port of Battletoads in 1991. As Machacek explains in issue 235 of Retro Gamer, "he actually asked me to port the in-production NES game. I suggested that, as I’d have to write everything from scratch anyway, if we did some extra design work, we could have a new sibling title for limited extra effort."

That experiment paid off, leading to solid sales for Battletoads on Game Boy. But if converting an NES game to the portable basically required starting over to make a whole new game, that would be doubly true for Donkey Kong Country, a technologically advanced game released years later on the superior SNES hardware.

As Machecek explains, "in 1994 when Tim asked if I’d do a DKC port, I argued the same case and he agreed, although the ‘bit of extra design work’ would clearly be more substantial here."

So Donkey Kong Land became its own, new title, built out of the artwork rendered for the original SNES game. Of course, the new game still suffered some cutbacks - only one character on screen at a time, limited bonus areas, and fewer ridable animals - but it expanded on the previous game's level design and introduced some stages, like Gangplank Galleon, that would be further explored in the proper numbered sequels.

Nintendo's portable would eventually get its own complete port of DKC when the game hit Game Boy Color in 2000, but technical limitations meant that we got a much more interesting follow-up years earlier.

There's much more on the creation of Donkey Kong Land and the world of classic gaming in issue 235 of Retro