I posted this at the link, in the middle of a discussion on why he wasn't cast in the film. That discussion may disappear soon but it's still there at the moment.
One possible reason: When rehearsals, pre-recording and then filming started in California (late April-early May 1966), Reilly was in Skyscraper.
Another possible reason: They may have thought he would look too old onscreen for the character.
Another: They may have thought his performance style would be too big. Even though it's a film with big performances, they may have feared that his style might just be too big to work and he might not be able to adapt. Morse had already made a number of films.