I honestly don't think the hold up is because Fox "isn't confident on the film" because, well... that doesn't make any sense. If you're not confident that your film is going to be a hit, you don't hold the marketing for that reason, you invest more and more on marketing so you convince people to go see the movie. That's pretty much what Warner did with Suicide Squad. And also... Remember Fant4stic? I seriously doubt that Fox had the least confidence on that movie, and it had regular marketing just like any other.
The hyopthesis that come to my mind are... Either they're using this growing anticipation for the trailer as a marketing strategy - smaller movies have done that before, but I don't put all my chips on that option -, or the reshoots just put them in the most difficult timeframe possible to start marketing the film, with Avengers being the hot topic of the moment, and their only possible way out of this is attaching the trailer to Deadpool 2.