the problem with most legenadries is that they're always in a group of two or more. If you add Zacian, the 9 million people who bought Shield will be upset, and vise versa. The third version legendary could work as a non-biased option, but I can't exactly see Kyurem or Eternatus working as a character. The only legendaries that I think could happen are Regigigas, Heatran, Silvally, Urshifu, and Calyrex.
The thing with Pokemon is that it's not above showing that type of favoritism.
Suicune is the queen of that. It got to headline its own game with Crystal, had a unique storyline in HGSS whereas the other two stayed as roamers, got in Pokken, and gets top billing in promotion out of all the Legendary Beasts with even an anime episode and Planetarium appearance happening in the past year.
Lugia gets the lion share of appearances including more anime episodes, anime arcs, merchandise, movies, etc. Whereas Ho-oh only had a recurring cameo that stopped after Gen 4, the I Choose You movie, and a couple other smaller appearances.
Reshiram got an anime arc and gets top billing in merchandise for BW whenever N is involved, whereas outside of Masters Zekrom has only gotten token outside appearances by comparison.
If Pokemon feels like there's more worth in pushing a certain Pokemon or form, they don't care if it's in a group. They'll push it as much as they want even if the others in its set get sidelined as a result. Zacian being chosen over Zamazenta wouldn't be out of the ordinary for the franchise, in fact, it'd be par the course considering the popularity gap between the two.