I would like to know those reasons, but as it may open an old can of worms I can understand why you wouldn't want to divulge them again.
I don't mind opening cans of worms :)
Remember I am talking about the most serious allegations here. I don't care about the minor ones.
So there are a few main reasons, really. First, the club wants all these matters settled once and for all. Secondly, it was calling the PL's bluff - you really want to do this? A third would be that the club didn't trust the PL to play in good faith.
Imho, the club could quite easily have played ball on third party evidence and provided to the investigation the same sort of evidence they (eventually) provided at CAS. They chose not to, I think, because, firstly, they didn't have to under the rules (no-one ever volunteers information in an investigation) and, secondly, because providing information to an investigation runs the risk, in fact the likelihood if the other party is acting in bad faith, that the information leads to further questions and requirements (once you have conceded to providing information, you can't really refuse to provide additional information).
So the club deliberately didn't provide to the PL investigation the sort of evidence that could have shut down the most serious allegations and that the investigators
knew existed because it had already been provided at CAS. Why? Because it forced the PL to either: face a humiliating defeat to their reputation by not referring the club to a disciplinary committee when there were serious investigatory issues still unresolved; or, proceeding in the knowledge that their evidence won't stand up against the weight of counter-evidence the club would provide (as at CAS) and face a humiliating defeat there instead. To shit or get off the can, basically.
The PL chose to shit out 115 unanswered issues from the investigation and here we are with the PL, in my opinion, about to face a humiliating defeat.
Everybody else on the planet thinks you don't enter into a legal process if you don't have to because there is always the chance, no matter how small, of an unfavourable outcome. I think they are underestimating the determination of Mansour and Khaldoon to teach the PL a lesson and their absolute confidence in the counter-evidence they have at their disposal.
Note I didn't say because they didn't do it. I think my analysis still holds even if they did.