For the exact reasons I mentioned. Madrox is and always was the lead character of the modern X-Factor series. The book and premise of the team was built around him and it makes more sense to have it be a spinoff from him than the other way arund. Comparing it to Avengers and JL is the wrong way to think about it bc none of the characters warrant a solo film outing to slowly build up the team.
The original Madrox series featured Siryn and Strong Guy as supporting cast members and the team was rounded out with Monet, Rictor and Wolfsbane when the team was set up for the X-Factor ongoing
The setup is right there. The Madrox film serves as the origin story and introduces half the cast with X-Factor spinning out of that jumping straight into the action with the remaining three. That format is better than cramming an origin story and introducing 6 main characters into one film