Well the Solari's leadership was destroyed almost completely by Diana, so it's likely going through some rebuilding at the moment. I mean at the end of both their lores it's pretty much made clear that both Diana and Leona want to find union between their people because they gained that ancient knowledge, problem is Diana doesn't know that Leona knows like she does, so It's on Leona to fix up that relationship.
Also let's not forget that these lores are written within the bounds of the knowledge the champions themselves have. No where has it been said why the Solari reject all other forms of light other than the sun. I mean think; you're fervently religious folks and a woman comes in talking about a faith that's basically the opposite of your own and how it can exist in harmony with yours, when that goes against everything your faith is based upon. Of course you're gonna be a little pissed off, especially considering the Solari exist in a part of the world with no other religions to contest them, so religious tolerance probably isn't their strong suit. While corruption from within is a possibility I'm more inclined to believe that the Solari actually hold the belief that the sun is the superior life purely based on time and the development of their beliefs.
Let me pose a totally hypothetical question: you're the ancient Solari order. Your long time allies in the Lunari go to war with you for no reason, and try to destroy your beliefs. What do you do?
With so little to go on about the ancient history of Mount Targon we can only guess as to where the Solari's religious practices come from, why the Lunari effectively died out, and why the Solari reject other forms of light.
