I think it's partly because in seasons 2 and 3, Lionel was at the height of his powers, and the interest in Luther family dynamics was high. Season 4 lost what made Lionel a great character. And season 5 is occasionally recapturing that, but mostly I'm confused. Is he Jor-El or Lionel? Season 4 let us down by not giving any firm motives for Lionel's behaviour, but making it entirely ambiguous and up to the viewer to decide if he was faking or not.

With seasons 2 or 3, I was a lot surer of Lionel's motives and his place in Lex's life. Most of the time I don't even know what Lex's feelings for his father are any more. Although it is surprising there haven't been any fics on Lionel trying to become part of the Kent family. Some interesting fic could come out of that.

And thanks for the recs!