It's a weird decision for the anime to push Mikleo to the background because Mikleo is one of the most popular Tales of Zestiria characters, if not the most popular in Japan. I wish I had a poll for this, but, from my experience, Mikleo is big in Japanese social media, discussions, fan art, merchandise, and promotions. Basically, he's a big moneymaker. I know this is my subjective analysis, and maybe I'm not seeing everything, but if Ufotable wants TOZX to be more popular, Mikleo helps.
From what I’ve seen here on Tumblr in terms of fanart, I’m inclined to agree. Mikleo is really popular, and it really is a strange decision to shove into the background in Zestiria X. I go online to buy Zestiria merch (both via auction and regular online stores), and Mikleo’s character merch always sells out first and is usually the most expensive to get on auction. Sorey’s is next, and anything featuring both of them is like … ugh, get ready to throw a ton of money at it, lol.
So it really is weird to me that they’d shove the most popular character into the background especially since Mikleo’s merch sells amazingly well and the best way to keep his merch selling is to give him screen time. But then I don’t get what ufotable is doing with the anime as a whole, so, your guess is as good as mine, anon.