Is it known why Roman Romanenko and Dmitry Kondratyev retired early in their careers as cosmonauts?
Romanenko retired for health reasons. I haven't found data on Kondratyev.
According to a newsru.com article (in Russian) , Kondratiev quit cosmonauts corps and started to work for a private company, not related to space.
Another article (also in Russian) contains an interview about Lonchakov's quitting the cosmonauts corps and also mentions that Kondratiev quit in protest against transferring the cosmonaut training center from military to civilians (Kondratiev is a colonel of RF Air Force).