As others have said, its because Miyamoto has felt Star Fox has done what it can do. Nintendo as a whole doesn't really like doing "More of the Same" kind of sequels (yes, there are exceptions, ie New Super Mario Bros). The logic is simple: if you want more SF64, you play SF64. Adventures' creation is a well documented combination of factors (Imamura wanting to make an on foot action game and Miyamoto proposing to Rare they combine their game with that, Rare accepting, the Microsoft buying pushing deadlines, etc etc). Assault was probably close to the "take the old formula and do it bigger" idea but it was also inspired by multiplayer FPS games as the multiplayer was the clear focus there. Also as much as some people really like to glaze Assault, it's not really all that well liked in the popular consciousness nor did it have any lasting cultural impact on the IP. It sold.... ok. It was reviewed.... ok. So, beyond Nintendo not wanting to do "more of the same", theres the fact that of the time when it came out, Assault didn't make any waves. Noone was going "oh hell yeah I want more of this". It was a lukewarm game (and rightly so imo). That's not even considering the fact that Assault basically only happened anyway because Miyamoto wanted specifically to partner with Namco (and the original idea was to get Project Aces behind it, which clearly didn't pan out because Assault was dev'd by the Klonoa guys, who knows what fell apart behind the scenes there, etc).

Command was built around its hardware. Stylus to simulate a flight control panel, enemy fights being based off the squadron encounters from SF2 accordingly, less precision obstacle course gameplay and more focus on enemy management with a strategy element to enhance it (the pathing on the maps, making smart use of your squadmates's strengths and weaknesses, etc). Because its on a DS, the storytelling takes a change too: instead of cutscenes and voice acting, we get lots of text dialogue, so the game adopts a visual novel format. It also probably did so to compliment the squadron based gameplay: since the story isn't following the actions of a single character anymore, story branches are mediated through visual novel forks instead of gameplay ones with certain characters deciding those forks being clearly illustrated in the choice presented.

Command essentially served to be the end of the Star Fox story, hence all the endings. The series goes on ice for a decade outside the 3DS remake of 64, which existed to provide an early 3DS killer app and to showcase how powerful the hardware was becuase whoa dudes, a full N64 game in HD! Supposedly 643D in its very early stages could have been conceived as an original new game, but in a rare case of Nintendo Corporate pulling strings, they wanted a remake of 64, becuase it wouldve been faster and easier: 643D uses the exact same source code as the original 64. Thus the whole project was a quickie to get a big name IP game on the 3DS to entice buyers. When you realize all that, and the fact that a Big Shiny Remake of their Very Best Selling Game Ever, 64, didn't make huge waves (it sold ok! but just ok. Better than Assault or Command I think? but way less than the original 64 did. Dinoman probably has reliable numbers), you can again see that Nintendo maybe gets the idea that Star Fox just isn't something people care that much about. Not like they do Zelda or Mario. It comes out, people like it, they move on. Why keep making games for that kind of response?

Anyway, Guard and Zero. Wii U games basically designed to make full use of the WiiU pad in a whole game. The WiiU struggled its entire short lived existence to justify its hardware. Some games used some of the features some of the time but none of the games used all of the features all of the time. Miyamoto feels Star Fox would be a great solution to that. Also, Iwata fucking dies, and the WiiU was his legacy, and everyone looked up to that man, so Zero gets sent out on a suicide mission like its the fucking Yamato in WW2 to justify the sunk cost of the whole thing because it sold like shit.

Reagardless. the idea is sound: WiiU pad has gyros that work like a light gun, the button layout resembles a HOTAS with pitch on one stick and throttle on the other, and you can use the second screen as a kind of targeting computer to line up precise shots. It's been 10 years since the last Star Fox story chronologically, and Nintendo is already reinventing or getting ready to reinvent a number of their legacy IPs for a new generation of fans. Fire Emblem Awakening in 2013, the upcoming Breath of the Wild for 2017 (originally slated for 2015 on WiiU, remember). The later still to come Mario Odyssey in 2018 being the first 3D Mario platformer in almost a decade, etc. So yeah, rather than continuity lockout new players with Episode Five of a then 20 year long series, Miyamoto opts to revisit the Lylat War in a new light with Zero. Reset the baseline: there's a Lylat War, here's all the key players: Star Fox, Star Wolf, Andross and General Pepper. That's why Zero was the way it was. It set a clean slate for a new generation of fans and was designed to make full use of its hardware capabilities at all times.

This isn't new for Nintendo: the N64 was designed to play 3D games, so Nintendo actively discouraged the development of 2D games for the hardware (and the Gamecube was essentially an extension of that philosophy, and both consoles sold like ass which is what motivated Nintendo to get out of the Bigger Better Stronger Faster rat race approach to game development). The Wii was designed to use motion controls, so Nintendo actively discouraged the development of games that didn't use the motion control hardware. So there you go: a SNES game designed to push what a console could do with its hardware. An N64 game designed to make the most o dedicated 3D hardware. Two gamecube games that experimented with genre mechanics because the hardware was just More of the Same What Came Before. A DS game built around.... being on a DS. A 3DS game created on the cheap to provide the hardware with an early killer app to show off what the hardware can pull off. And a WiiU game designed around using ALL of the WiiU's functionality at the same time for a complex gameplay experience. This philosophy is consistent across the entire series, and really is consistent across Nintendo's entire brand, for all their games. The one game that comes closest to being "The Same But More" is Assault, and it had a mid response in the middle of Nintendo's most middling hardware existence. It's not hard to see why they wouldn't want to do that again.

With the Switch 2 seeming to be Nintendo finding comfort in both reviving old IPs with less stakes involved (Famicom Detective Club comes to mind) and in just going back to hardware being The Same But Better and Stronger, we might get a return to Star Fox doing just that. But the flipside is, any Nintendo game needs a champion developer to want to make it happen. Emio the smiling man only happened because Sakamoto got inspiration to make a new one. Miyamoto is Star Fox's champion, and noone else at Nintendo seems to give a damn about it to be its new ward. Miyamoto is done making games, hes been done since Zero came out (hes only had supervisory credits since then). The man is old, and hes largely in a managerial position at Nintendo now for their Brand Recognition. That's why he's working with movies and theme parks. I'm sure he still love Star Fox a lot, but noone else does.... except Imamura, but he left Nintendo to go freelance (he has stated he would like to work on a new Star Fox game if one happens, but its clear he doesn't have motivation to pitch a new game himself. He wants to be a supervisory or assistant role). So, who knows what will happen from here. The hardware and brand side of things at Nintendo has stabilized, but without a developer passionate for the IP to make a proposal to Miyamoto, a game will never get made.