Possible explanations: They have no idea about the game's direction. Retro studios just doesn't want to do another DK game. No other team inside Nintendo wants to do a DK game. They believe DKCTF is good enough as the Switch representative of the franchise. The teams are rather focussing on games they have closer to their hearts. DK became fat from too many bananas.
In game development, you need to have a vision first, then create the concept around that, then assemble the manpower around that. My best take is that nobody came up with a vision for a new DK game. If you forced a new game without a focused direction you'd end up with a haf-assed product that fails to meet expectations and damages the franchise. I'd rather not want to have half-assed games and instead accept no game at all, but that's just my fries threwn across the table.
