Metroid is amazing. But it's not something that attracts a big audience, let alone on a Nintendo platform.
Metroid Prime 4 could do well if they ever finish it. It just doesn't have the same 'nostalgia' as Mario and Zelda and it isn't easy to reinvent like Fire Emblem.
I think Metroid's biggest issue tbh is that it doesn't appeal too much to most nintendo fans. Even then it's not an AWFUL seller, it does pretty well just not Zelda (which only did said numbers on specific games that ended up changing the landscape like OOT, BOTW and the OG with few exceptions) and definitely not Mario and Pokemon numbers.
Doesn't help that it's prone to long ass gaps between releases, and two out of the three latest games got massive backlash. It's also kinda mismanaged. First 2D Metroid since Zero Mission comes out on the 3DS when the Switch has crazy momentum? Yeah Samus Returns wasn't put in the best spot either.
That said Nintendo does seem to want to get it right with Prime 4 at least. So at least there's a game on the horizon though I doubt we'll be seeing it anytime soon.
It's a shame since Metroid is super appealing to me - the atmosphere, the isolation, the art direction, the music, and getting lost in these giant obtuse alien mazes... it's not bogged down by loot systems or RPG mechanics like many other Metroidvanias are (for better or worse)...
Of course there are Nintendo fans that lose their shit over a new Animal Crossing, Mario Party, or Pikmin and I'm just staring blankly...