You guys are missing something critical... people do use Zacian on sun teams. However, the reason why has nothing to do with Solar Blade, and everything to do with Zacian just being that good of a Pokemon.
If you've played Ubers or AG anywhere above the lower ladder, then you will know that Zacian-C is one of the most prevalent Pokemon in both formats. Sun team or not, the Pokemon is going to get usage because it's simply too good of a wallbreaker and end-game sweeper.
A little bit of extra damage from Fire attacks does not change the fact Zacian-C outspeeds the entire unboosted metagame, has an incredible typing, and goes to +1 upon every switch-in to go with its base 170 Attack. Venusaur, Charizard, etc. are mainstays on sun teams, but they don't get anywhere close to replacing Zacian.
If you won't take it from me, then you can take it from the stats. Zacian-C features on 61% of high-rating Ubers teams (that is all teams, not just sun), and on 52% of high-rating AG teams. The Ubers stats also show that Zacian-C is used 18% more often on teams that also include Torkoal (which itself sits at 9% usage). If you're playing AG, then you're probably not going to see sun teams in the first place, because Pokemon like Torkoal and Venusaur are so off-pace with the others available in that format.
Solar Blade does clock in at 7% usage on non-crowned Zacian, but again, it's not the difference-maker for its viability on sun teams.