Because if this Geoff ever comes clean, then Lloyd has already painted a picture that Lloyd was the victim here.
And here's the rub... It might not be necessarily Geoffrey he needs to worry about. Lloyd gave details of Geoffrey's misdeeds, including him marking his own sister for dating before her divorce was final and enforcing it on people who were seen anywhere with her, including Lloyd. Anyone who went to Bramhall or a nearby congregation reading his book would know PRECISELY who he was speaking about when he says "Geoffrey".
His singular focus on Geoffrey "poisons the well" as sbf said, so if anyone who knew about it and confronted Lloyd, he had a ready retort. "You must have heard those lies from Geoffrey, and I documented all the reasons why he has an axe to grind against me".
The fact that he does turn up the volume on Geoffrey lends credence that what he did was known in Bramhall. If not, why worry about being blackmailed?