Why would people want the newer characters? That's like wanting Diddy over DK
The older DK isn't in Smash though. Its the second DK, who's design comes form DKC, which is when Diddy showed up.
Really now, there isn't any true face to either the GS or S&P series, because they keep changing protagonists. There's just a first character who seems to get all the preference in these sort of debates. Now Sakurai is actually sort of unpredictable when it comes to these things, since he has second generation characters (Like Ness and DK) and first generation characters (Like Marth and technically some of the later Pokemon decisions) but if I had to make a guess, its based off of where the series is at the time he decides the roster. Fire Emblem had been dead for a bit at the time of Melee, but was getting a revival, so it had both Marth and Roy. The Mother series wasn't really dead at the time of the original Smash, but Ness was chosen since Lucas couldn't be at the time. The original DK had been passed up a long time ago, and the current one was the DK that we mostly all know, and he was chosen. Gen 2 hadn't come up in 64, was still linked heavily in Melee, and had remakes prior to Brawl, so it was relevant all throughout.
Thus it would actually be interesting to see how GS and S&P fair (If they get anything) because they aren't old series, and they have updated recently, but with both its the future generation which is essentially repeats of the previous.
Now that still leads me to the Saki thing which has been bugging me, since people seem to be justifying him through Isaac, even though Isaac is totally different/hasn't been included. Isaac has had three games, not one, and his series always was international. Saki started off as Japan only, and then had a digital download release + AT which then pushed his series into getting a sequel that went international, though not with him in it. That being said, is there ANYTHING that justifies him over Isa outside of being first?