Yeah, a lot of Nintendo's franchises seem to sit around for quite a while. Donkey Kong Country was dormant for 14 years since Rare was bought by Microsoft (not that I know if they had future plans anyway). There's a 10 year gap between Star Fox Command and Star Fox Zero, except for a SF64 remake on 3DS. Not to mention Sakurai reviving Kid Icarus after 20 years.
In that regard it's not that surprising that things like F-Zero and Custom Robo have gone silent, considering they were never really on the level of DK or Fox. Especially F-Zero, Nintendo could easily say that they have Mario Kart, and any other racing games they make just aren't going to be able to compete in terms of sales, so why bother. There was Fast RMX, but it's only on the eshop, as far as I know.
Heck, they don't even put Captain Falcon in Mario Kart, even though they put in characters like Link, Villager, and Inkling into Deluxe.
Custom Robo is a little different because unlike F-Zero, I can't really think of anything that really fills that genre mixture of RPG and arena fighter. Maybe it's just too niche? It seems like Nintendo Franchises only get so many attempts before they're scrapped. And when you look at the era both franchises existed in, which was largely the 64 and Gamecube era (F-Zero did exist earlier, obviously), Nintendo was just getting completely outsold by Sony. I never even realized how badly until looking at the numbers. Of course, Nintendo would never admit that. Or at least, they're not looking back. So you really do just have to hope that someone from that era of gaming grew up and thought "Why do these not exist anymore?" and will push for them from inside the company. Barring that, there's not a lot of reason to expect they'll come back.
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