If Dixie didn't make the base roster, let's look at who did and why:
Inklings, huge new first party franchise and a success, no brainer
Ridley, huge request dating back 15 years, main Metroid villain
Simon, the next big third party, dates back to the NES days and Castlevania heavily associated with Nintendo
King K. Rool, huge, huge requests, the votes for him also likely took attention away from Dixie
Isabelle, she became the main mascot/face of Animal Crossing
Assuming the leak is real:
Geno, huge decade+ requests since Brawl, Sakurai himself wanted to put him in for a decade
Isaac, huge decade+ request since Brawl. He seemed to score very high on all the fan ballots online so he likely had a lot of votes
Chorus Kids, the Gematsu leak implied they were planned for Smash U but cut likely due to 3DS limitations, so Sakurai finished what he started
Mach Rider, just the next retro NES character. A Sakurai personal pick
Banjo-Kazooie, extremely huge character considered a shocker due to being owned by Microsoft, big request, etc.
Not counting the echoes, a lot of these were also huge requests going back decades or Sakurai personal picks. I think Dixie just got unlucky, as said a lot of the DK fanbase went for K. Rool over her so that took away a lot of her votes. But if the other leak is true then Dixie was planned for Ultimate anyway, which means she might have started as an echo but then been delayed because Sakurai couldn't make her work as one. So it would be better if she's more of her own character for DLC.