It lists the reasons for denial as:
"Your refusal to accept responsibility for the offense(s) committed," and
"The negative recommendation made by the prosecuting attorney."
It appears that Mike maintaining his innocence is seen as an attempt to not accept responsibility, even while there is evidence that corroborates that the shot was fired from a location where it is well known he was nowhere near. This phenomenon is referred to as "the innocent prisoner's dilemma" by law professor Daniel Medwed who asserts that it is unfair and unethical to require a prisoner who may have been wrongly convicted to provide false admission of guilt or remorse.
In regards to the negative recommendation made by the prosecuting attorney, this is outweighed by the fact that the officials at SCI Graterford, where Mike is held, have recommended him for parole. These are the prison guards and personnel that he has contact with on a day-to-day basis as opposed to the prosecuting attorney who has had no contact with him at all in decades.
