We know that Lord John Grey was “exiled” to Ardsmuir Prison in Outlander. We get a few details as to why that was in Lord John and the Private Matter. The start of the Lord John Grey novels ...

Lord John Grey grew from a side character in the Outlander ... The books mostly take place during Voyager, set between Ardsmuir closing down and Jamie going back to Scotland.

They had no appetite for a prequel that wasn't tied to any particular book, particularly when the character of Lord John Grey was right there and already had a number of spin-off stories centred ...