No one - Manning, Gruden, or any competent person - wants to be put into a position to fail. This might mean those people aren't ready, or don't feel they have the "right stuff" to succeed in that position. It might mean that they feel the fit isn't right. It might mean that there are too many obstacles to overcome, in their opinion. Lastly, it might mean that it "just isn't worth the trouble for the money". Obviously, we don't know if any of these reasons apply.
What we do know is that - while seemingly trying to score a "home run" - things got out of the proper sequence for peoples commitment. If Gruden was ever to come aboard, it would only be after Manning had tacitly accepted being PM, then Grigson terminated by Irsay, then Manning named PM, then Manning terminating Pagano and starting a coaching search, then let Manning - not Irsay - lead the discussions with Gruden (or coaches of their choosing), then the hiring of the HC (say, Gruden), and then filling in the rest of the staff, replacing those that need replacing.
There is a generally accepted hierarchy to these decisions necessary to assure highly competent people that this is the right organization doing things right.