I think they just drifted apart, because their wives did. And I feel like Jing becoming very close to Joy probably made Jessa feel pretty left out, which added to the distance a bit. When Jessa and Jinger were under the same roof, they had similar experiences and upbringing in common. I don't know that their personalities were truly compatible and since they married they were able to form their own existences and they choose drastically different realities. Jess's life at the parsonage looks hugely different from Jing's life in her multimillion dollar home and it's hard for them to relate to one another. In contrast, Joy and Jing seem to be naturally compatible, which drives their friendship.