Star Fox, along with F-Zero, is one of the underrated Nintendo franchises IMO. They seem to want to shove Mario and Zelda down our throats every time they get the chance, but the other franchises are left aside.
Look, I love F-Zero as much as the next guy (not so much Star Fox, though), but do you really want to ask that question? The answer is obvious, sales. Star Fox and F-Zero sell CRAP compared to Mario and Zelda.
And does it matter if they milk Mario and Zelda if the games are still up to quality? What's there to complain unless you're not into the genre. (In which you should speak for yourself.)
And I also don't get this "other franchises left aside", with the Wii and DS there were:
-2 new Wario games
-A new Star Fox
-A new Sin & Punishment
-2 new Rhythm Heaven games (one of them being the first to be localized in the series)
-5 Warioware games
-Around 5 new Kirby games
-A new Smash Bros
-5 Donkey Kong games (one of them being a revival of the Donkey Kong Country series)
-2 new Yoshi games
-3 Fire Emblem (albeit one wasn't localized)
-2 Chibi-Robos (one wasn't localized as well)
-2 new Advance Wars
-2 new Animal Crossings
-New Tetris
-New Picross
-New Starfy
-4 new Metroids
-new Golden Sun
-new Punch-Out!!
-Obviously Pokemon
-A BUNCH of new IPs (Hotel Dusk, Ouendan, Another Code, Elite Beat Agents, Wii brand games, Brain Age, Nintendogs, Meteos, Xenoblade, Last Story, among others)
And then with the 3DS and Wii U we have:
-A revival of Kid Icarus
-A new Luigi's Mansion (No, I don't consider this a typical Mario game)
-new Pikmin
-new Bayonetta
-new Fire Emblem
-new Animal Crossing
-a remake of a Star Fox games
-A new IP in Wonderful 101 and X
-new Warioware spin off
-new Donkey Kong Country
-other stuff i dont feel like remembering
among other things. Hardly shoving down our throats. Or maybe it seems that way because people are ignoring all the above titles (most of them sold bad) despite all the whining. :/ Oh, and they're quality games. Elite Beat Agents is better than pretty much 95% of any game I played.
This is coming from someone who considers Wario Land and Rhythm Heaven his favorite Nintendo franchises, which are pretty much niche. Even more than F-Zero and Star Fox.
To be honest Star Fox isn't much of a loss with how there hasn't been a great game since 64 (and even then I think 64 is a bit overrated as much as I'm enjoying it right now). F-Zero is a much bigger tragedy.