Well I think there might be something missing in your screenshot because Fisher states in his letter that he received Shewmon's statement from Grillo's court. So it seems it must have been noted to exist there at some point.
http://www.thaddeuspope.com/images/Fisher_letter_and_Objection_to_Fisher.pdf
It's not missing. Fischer received Dolan's writ of error corum nobis, which included everything Dolan may or may not choose to use as evidence at the hearing. Courts are kind of funny that way. "Nothing exists until the court adds it to the record."
What does that mean? Well, in court there's this thing called, "Discovery." Where one side (typically the prosecution like you see in movies, or in civil court, the petitioner) must provide to the other side every single "thing" - or piece of evidence - the petitioner has that MAY be brought up in court to use as evidence to prove a case. Special attention on the word "MAY". Just because it's provided, doesn't necessarily mean it's actually going to be used in court. Court isn't really like the movies. One side can't actually "surprise" the other side with some completely damning or discrediting piece of evidence. It has to be provided for the other side to get a chance to refute it if need be.
When it comes to expert witness testimony that may or may not be used as evidence to prove a case, a specific process must be followed in order for the court to even allow that person to speak as an expert witness. Dolan knows this process as an attorney very well. A letter from that person included with the initial filing isn't binding, just as much as the petition its self isn't binding - it can always be withdrawn before going to court, or not even brought before the court at all. It merely just informs the other side that this is "evidence" we have in our "big treasure chest of evidence", and MAY or MAY NOT present in court.
When it gets to the point that an attorney actually, truly considers wanting to have that person testify to what they swore in a letter, that expert witness must then request with the court to go on record to be admitted into the court room AS an expert witness. In the rule of law, "best evidence" is always preferred in court. What that means is, if a better form of the evidence you are presenting exists, then you better present THAT specific form of evidence or have a dang good reason why you are not able to. In the case of a "letter" from an expert witness, that form of best evidence is actually verbal testimony from that person's own mouth while sitting on the witness stand under oath. This sort of legal requirement is often a problem in cases where they use a video recording to show a person's face while robbing a bank, etc. The court requires the prosecution to present the ORIGINAL CD, tape, etc of the robbery, not a copy of it. "Best evidence" is the original, a copy is a copy. Sure, a copy would eventually be admitted if the original no longer exists for some reason. But not so long as that original does exist.
Which brings us to: why would someone write a letter as evidence, but then not follow through with a request to be admitted as an expert witness via court filings? Well, they can't be held in contempt of court for lying, if they never present themselves to testify before the court. Nor do they really risk any real economic sanctions such as revocation of their medical license for testifying in a court case wrongly as an expert witness...if they never testify at all.