Anyway, this drought of games isn't good. I was really hoping for Nintendo to re-establish DKC as one of their main franchises after DKC Returns, but I was awfully wrong there.
The DK situation is a bit odd because as far as making it a significant franchise, they both have and haven't.
In terms of numbers, they have released 4 games on 4 different systems in the last 10 years which is a pretty solid record. The issue of course is that two of those titles were remakes of previous ones, meaning that in terms of new content its really been a pair of games in the last decade. While I don't think there's any one reason for it, there's a couple of factors that explain it a bit.
Part of it comes from an earlier pattern I've noted of Nintendo being less willing to release similar types of first party games alongside each other. The 2D platformer is a bit more lax in that regard, but its worth noting that every recent DK release came out in a year that a NSMB or Mario Maker game
wasn't coming out (2010, 2013, 2014, 2018). Could be a coincidence, however it suggests to me that the company is not keen on having the genre compete with itself. That right there might have slowed down some of the potential production of further DK games at certain points.
There's also the factor that in an odd way, the two Country titles were a victim of their own quality. Returns was a lauded game whose biggest weakness was the motion controls, and Tropical Freeze was even more acclaimed release that suffered for being a Wii-U title with lackluster promotion. With those facts in mind, there is a clear logic to porting them to systems where the controversial controls weren't a factor and/or that had financial momentum to them. Its a wholly intelligent move to save money on development with the downside only really being for the fan (that we're getting less DK games overall).
One can't also forget that Retro was the go to developer for further games and was apparently heavily occupied with some kind of project/s after Tropical Freeze, so much so that not even rumors of another DK game even seemed to emerge. With them being the best possible team to make the mainline games, if they couldn't do it, its not that hard to imagine Nintendo believing that no one else could.
Finally, any titles beyond mainline games were less likely because of timing. The last decade has seen a lot of side-games (outside of the big three of Zelda/Mario/Pokemon) gradually fall by the wayside or end up fairly lackluster. Kirby was a bit of an exception, but even its fallen into free to play games in the last couple of years. The Wii-U era attempts at such resulted in titles either disliked (Amiibo Festival, Federation Force, Chibi Robo Zipline) or outright forgotten (Hey Pikmin, Mario vs DK Tipping Stars). Between the costs of HD development and the inability for Nintendo to get much momentum for their mid-level franchise spin-offs, DKC the last decade ended up being IP both simultaneously ideal for building on
and not practical for expanding.