But yes, of course there will be reasons that are more understandable to himself, because I personally am not sure if the reason is to focus on Manchester United that makes Varane choose to retire from the national team.
After previously Benzema decided to retire, now Varane has decided his future from the French squad. I don't know if there was actually an internal conflict or if it was decision that proves it's time to give other players chance. But for whatever, hopefully it's the best decision for Varane.
Now there is only one choice, Varane only wants to focus on club level even though he has won many titles with Real Madrid. I don't think his decision to retire from the France squad will see him fail at club level as nothing motivates him anymore like being called up by the national team, but Varane is expected to give everything at his current club.