I only have one theory in mind about that. I think Fox is really confident in XF, they think it’ll work well with or without promo because the brand is strong enough. I believe Disney will buy Fox eventually next year, and for that, Fox needs keep their ratings at their best in the first few months of 2018, or at least to not downgrade them. Disney may have had thrown a ratings clause into the negotiations. The series Empire which is the one with the best ratings on Fox airs on Wednesday and stops right before the Christmas holidays. So I think they needed to replace it with safe bet right after the Christmas break.
That would explain why it feels rushed, why Wednesday, why we don’t really have new promo pictures, trailers, and why they’re not even going to do an official premiere night. I’m still hoping the promo will happen in January anyway, after they air the first or second episode. That would still make sense as there’ll be 8 episodes to go.