Also, being so closely associated with one character / genre can really be a double edged sword for actors.
If he wants to flex his acting muscles a bit more, now that the show has wrapped up, it would be a good time to step away from those genres and try to shake the…ahem, Snow, off of himself.
Away from his time on Game of Thrones, Harington has almost exclusively done historical and / or fantasy films and television shows that have often kept him in the heroic Jon Snow vibe.
I’m sure he won’t forget about his time as Jon and what the role did for his career, but it makes total sense for him to want to distance himself from the show and any potential roles that wouldn’t allow fans (and directors, for that matter) to see him clearly as a totally different type of character.