This is kinda because Skull Kid is now the "expected" Zelda newcomer. Much like what happened with King K.Rool and Dixie before K.Rool was confirmed in the game.
And many people automatically assume that Impa is gonna be a Echo, and that's kind of a shame. She gets shot down before serious consideration, and some might feel she's a "forced Zelda rep". But yeah, can't take that away from the fact that Zelda is one of Nintendo's most popular money making franchises, got a whole lot of popularity because of Breath of the Wild being so revolutionary different than other 3D Zelda games, and that Impa has been with the franchise since it's beginning, and even now STILL appears!
Young Impa might not show up as much as Old Impa, but Ganondorf also doesn't show up as much as Ganon.. see where I'm getting at? The most popular form of the character can still be entering Smash Bros. and I really hope that Impa does get in. She's now relevant enough, popular enough, and important enough.. and it's about time for Smash to have a Zelda newcomer that isn't a version of Link or Zelda, or a one-off (or combination of both
). Skull Kid also meets these merrits, but honestly.. I think he's a little "shaky" for being tied so much to Young Link, and it would be BEAUTIFUL to have him now but.. if next game cuts Young Link, I think Skull Kid doesn't quite "fit" as much as before. Whereas Impa does, especially Skyward Sword's Impa as she's a known time traveler anyway, and would therefore make perfect sense to accompany any future version of both Link and Zelda.
I'm still 100% down for Skull Kid, but I won't lie that I prefer Impa and am more rooting for her. And respectfully, I know that the many Skull Kid supporters think the same, if we came down to it. And I don't wanna be dishonest about that. Zelda needs it's newcomer, but I personally, prefer Impa, and that's fine. People can prefer Tetra, Toon Zelda, Pig Ganon, Yuga, Hilda, a BotW Champion.. and am fine with that.