Moores' predecessor Sir John Smith was a successful self-made businessman and top sports executive (at various times a member of the FA Council and the chairman of a Govt.enquiry into the state of lawn tennis). His successor Moores was and is an amiable nobody who had tried and failed to make it in business (including his family's own business - how hopeless would you have to be not to succeed even there?) and, on his own admission years later when he was under pressure to sell the club, needed the chairmanship to keep him occupied. Small wonder we began to slide when he took the chair.
Parry's a qualified accountant and had done well in charge of the Premier League, but was never the man to take this club into a new era. For one thing, being in charge of the Prem.League was much more of a bureaucratic job than that of CEO of a major business concern. It doesn't necessarily follow, and certainly didn't in this case, that somebody who can perform well in the former type of environment will be able to do so in the latter as well. For another, Parry concentrated all the power at the club in his own hands and tried to make himself indispensable, a dangerous approach at the best of times, never mind if your suitability for the role is questionable in the first place. Last but not least, the boss who appointed him (Moores) was too incompetent to notice the need for change and too weak to do anything about it even if he had noticed.