Style and tone of the book, most of Fitzmartin's stories tend to be more horror-themed, and Rossmo's style might suit that more than Ortega's.
Ability to keep up with the schedule for an ongoing, I don't know if Ortega is doing other work and may not be able to commit to this book, but recently a few books are having these problems with some even being delayed because artists cannot keep up the speed, Rossom has shown that he can.