Kid Icarus is getting as much love as it deserves, it was a fantastic game back in the 80s (honestly, I find it infinitely superior to pre-SNES Metroid) and now it's like one of the best Nintendo handheld games ever created. (Subjective as ****, but I don't think there's anyone who can deny that it's a well-polished, high quality, succesful game that caused quite a bit of noise when it released)
The series may not have too many games, but they've all been ****ing great, plus, Pit is both a retro character reference, and a recent, very succesful game, so his series has more than enough reason to get a wide variety of content. And that's not to mention that KI items just work for Smash, I won't deny that there's other things in other series that would work just as well or even better, but that's another issue entirely.
And that entirely different issue is Donkey Kong/Metroid.
Do Donkey Kong and Metroid deserve another character? **** yeah, they do, they're ****ing video game giants, but you can't put the blame of it not-happening on Kid Icarus content. If it doesn't happen, it was likely not meant to happen, Kid Icarus or not.
So yeah, the issue's not that Kid Icarus is getting too much attention, it's just that other series are not getting enough (as far as we know.)