Because Sheik and Impa aren't that much alike to begin with. Sheik also was basically the most popular playable character in Melee and top tier. She also provided a super unique moveset. Impa has her own materials and was finally given something of note later on. Besides that, Zelda was really really bad without being able to transform into Shiek to somewhat alleviate it, so entirely separating them made it worse. She couldn't really call upon a decent move like the Phantom in Brawl either. Too early for that concept. So separating them would've made Zelda go from already unviable to possibly worse. That said, it's possible Zelda was planned as Sheik in TP, but we don't know if she was meant for that and scrapped or if Sheik got a remake in the TP style specifically made for Smash. Hyrule Warriors is quite vague on it. Either are possible.
To get back to Impa; nothing made her stand out at the time(it wasn't till SS she started to gain anything at all. You know, by the time 4 hit? No way she would've been in Brawl). Zelda and Sheik were a package deal. Neither had a consistent ability set, so the transformation was the core gimmick. It's only later that Zelda coincidentally had ties to the Goddesses(loosely, via SS), so her specials kind of worked out. But back then? Nope, it wasn't enough. And we already saw that even with her being separated, it barely helped. She's still very bad in 4. In Ultimate, she was buffed, but it still didn't change her issues. Also, keeping in mind Sheik was shown first, she's pretty much treated as being in before Zelda. It wouldn't even be that surprising if she was chosen first, due to being Zelda's fighting form, something that was barely a secret by the time Melee came out, and the only time in any canon series she could feasibly fight. Sakurai couldn't really depend upon the CD-i games or the TV Show, since Zelda basically was Link in every way fighting-wise. With the abilities from OOT and imagining her as a unique style of "monk"-like fighting(energy kicks, etc.), he found something... usable. He already viewed ninjas as very easy to transfer over, which explains how easily Sheik was added in general. Or why Greninja was easily chosen over the other ones(A fiery magical fox was also easy to use, but Chesnaught was... a bit less interesting. Since it didn't stand out nearly as much among a lot of characters who can already punch rather well. Though to be fair, Little Mac also has a clear gimmick, which is why despite being a puncher, he does stand out enough. Whether one likes the gimmick or not is irrelevant).
Last thing of note to Impa; SS provides her some abilities, but it doesn't mean it provides a clear playstyle outright. She's already unique and would be a pretty bleh replacement for Sheik even during Brawl since unlike Toon Link, she wasn't simply promoting a new game. The only character of note at that point was Impaz. She missed the boat by not being reused as is for any new games. Her designs are inconsistent too. The only actual relevant design was her larger one from the Oracle games, who is nothing like Sheik either, being a powerhouse character who has more in common with Donkey Kong at that point. So that was a no-go as well(that, and we didn't really get a lot of Capcom Zelda stuff for a while, so it's possible even if she was considered, there was some oddities with copyright/licensing. I don't think there was, but it's really hard to say what was going on). You can even tell how little Impa was on the radar when she didn't even get a Trophy at all during Melee. She finally had something in 4, which took a while. At least in Brawl she wasn't relevant outside of the Oracle games. The original ALTTP Ganon suffers even more, having zero mentions despite being the most iconic design besides arguably the first game at best. They obviously don't care that much about referencing specific games all the time. ...But being Ganondorf's only appearance with the ALTTP Ganon is loosely in Four Swords Adventures(Ganondorf is mentioned, but doesn't physically appear), that's not that odd.
But you might get the idea by now; relevance played a lot of roles in which to choose. At Smash 4, we got no new Zelda characters(compared to Melee and Brawl introducing new ones), with Ultimate only returning one cut one for a pretty large total of 6. Sakurai's choices aren't even clear to us why he doesn't go outside of the Triforce Trio. You can somewhat justify ALTTP Ganon not being there since regular Ganon is already a boss anyway(as massively different as they are), but the lack of any kind of Spirit still remains a mystery. Maybe TOTK will give us something. Maybe it won't. Maybe Ganoncorpse(nickname) is actually Ganondorf. Or maybe it Phantom Ganon. Or just a coincidence. Seeing Ganondorf and Ganon separated would be neat, though.
With 6 total characters anyway(Toon Link and Young Link clearly weren't meant to be the same, but just have a very similar role at that point), there was pretty much no need for more characters. It doesn't matter that some of them are clones either. It's still a lot of characters as is, and outside of Ultimate, it had 5 characters total for multiple games. Plus every game after Smash 64 had tons of content(multiple stages, tons of music, items, and they even had various enemies appear in more than one game or at least multiple AT's). Though also keeping in mind that the not many franchises even have more characters(Super Mario, Pokemon, and Fire Emblem), but with caveats; Fire Emblem has a bit of a lackluster content. While it has good music, it has no items beyond AT's only(and not many at that), most of the stages are generic(and it didn't even get one in Melee for that matter), and even then, it only has two Echoes and one semi-clone. Which two of them were directly based upon Marth(one deviated more), and one that partially relates to it).
