Fox and Falco are the most popular characters in the game. This gives them an advantage in resources over the other characters (that advantage being population) and as a result, they have a stronger pool of players who are developing their strategies and representing said strategies at the highest level of competition (aka the forefront of the metagame)
So while Marth and Sheik have approximately 3 reps each and Pikachu/Peach/Puff have approximately 1 to 2 each, Fox and Falco are consistently being represented by 8+ reps (Mango, PP, M2K, Leffen, Hax, Westballz, Fiction, Lucky, Colbol, SFAT, etc.).
This isn't to say that higher representation inherently means higher metagame development, but this appears to be the case since different strategies are being utilized. Mango, Leffen, and Hax are capable of representing the same character with noticeably different styles while still performing at a similar level of play. The same can be said for Mango, PPMD, and Westballz in regards to Falco. What this means is that there are more winning formulas on display, and thus, more information available to players who are trying to understand Fox or Falco in relation to the metagame.
Compare that to Marth, Peach, Puff, etc. who have fewer reps utilizing strategies at non-similar levels of play. Do we really understand these characters as well as Fox and Falco? I can clearly recall many players doubting the capabilities of Marth until 2013, and which definitively changed in 2014 when PPMD brought Marth back into "top level" so to speak. Prior to that, we had up-and-coming PPU and M2K's "FD counterpick Marth" which practically hadn't progressed since 2010.
Of course, I will admit the possibility that many characters could be linear enough to the point that the sole players who are representing x character at top level are representing the only working strategy they are capable of, but I would like to think that most of these characters are like Marth and that the main thing holding them back is the players (or lack there of) using them.