Mainly because I think I have a good idea of what Marvel Studios is looking for.
While Scott Derrickson is leaving the project, he’s staying on as Executive Producer, which implies an amicable parting with Marvel.
Now, the title ‘Executive Producer’ is potentially as meaningless as “creative differences” but assuming that relations between the director and the studio are good and the title is more than ceremonial, Derrickson may have had a say in who replaces them.
And that director is likely to be…Ciarán Foy.

He’s worked with Derrickson before, directing the sequel to Sinister, Sinister 2 though prior he directed the awesome Citadel (currently on Vudu. Check it out) and more recently, Eli (on Netflix).
Most importantly besides the prior relationship with Derrickson, Foy can play the role of the journeyman director while bringing a sense of atmosphere and tone that would fit well with the esthetic established by Scott Derrickson.