Then why didn't he stand and fight the evil, as a true leader would?
The plausible explanation is that he was complicit and the Lavender Mafia could prove it.
It’s all conjecture at this point. I would think standing up publicly to threats would be a good thing, but I know -zero- facts about this case.
Another plausible explanation was that he was an old man who was basically an academic theologian, which is not a particularly useful skillset for running a major institution in a time of crisis. He acted in what he saw as the best interest of the church.
Yours is A plausible explanation, not THE plausible explanation.
Another, IMO, MORE plausible explanation is exactly what Francis suggested, he was old, tired, and lacked the strength to continue in his duties.