That's the problem. This article sums up very well Nintendo's reliance on maximizing profits direction and how bad it turned out:
http://www.dromble.com/2013/12/10/nintendos-obsession-with-finding-one-game-that-can-sell-consoles/
That's the usual interesting but ultimately flawed article by Emily Rogers.
She doesn't address why the entire traditional industry is moving toward a consolidation into few big games effectively killing the mid-budget releases (said that next Retro project cannot be another DKC...).
"Nintendo is sitting on a gold mine of intellectual properties. Microsoft and Sony would kill for your intellectual properties and franchises."
lol Emily, they would for some of them (like Mario or Pokemon) but do not for even an instant think Sony or MS would treat better those IPs.
Just look how Sony and MS handles their own legacy franchises...
Because the latest games of the genre were disappointments with awful reviews/reception or were niche franchises with little appeal unable to replicate the same popularity from Star Fox/F-Zero?
It's irrefutable those genres (the whole genre) declined from the heights of the 90s.
Publishers aren't funding those kind of games anymore because they don't think they could turn a profit big enough to justify the investment.
Would a new Star Fox or F-Zero game probably sold more than a no-name in the same genre?
Of course but that wouldn't change the sorry state of the genre.
Nintendo has tradition in reviving old franchises and making them relevant again. They did that with Metroid in 2002, DKC in 2010, Kid Icarus in 2012 and Pikmin last year. I'm glad they didn't followed your "It ain't the [insert decade here] anymore" excuse, otherwise we would never see them coming back, or even play the great games which brought back their relevancy.
Nintendo treats really well their franchises, that's why I think (hope) a new F-Zero is still in the card.
It's fantastic Nintendo greenlit the Punch Out and X sequels on Wii and DS.
Retro, Next-Level and Platinum (if Nintendo keeps their current relationship) would be great options for Star Fox. Monster Games could handle F-Zero, they made a good job with Excitebots and Excite Truck. Now considering the budget worries someone point as a reason to not make F-Zero, Shin'en often works with low-budget titles with amazing results, so they would be an excellent financial viable option.
To be clear I'm not saying Nintendo will never release new games in the Star Fox or F-Zero series.
I'm just saying it's obvious why Kirby got more games released in the past decade compared to those other two series and it had nothing to do with Iwata and everything to do with series sale potential, how costly they are to produce and if Nintendo feels there are reliable teams that can develop them (like HAL or Retro).