Kristin Kreuk and Michael Rosenbaum were series regulars from the start, while John Glover’s Lionel Luthor was a recurring character in season 1 before receiving a much larger role starting in Smallville season 2.
A few episodes before the Smallville season finale, Lionel was assassinated by Lex in a moment that solidified his transformation into the evil DC Comics villain.
As for Lana, she disappeared in the Smallville season 7 finale, with Clark (Tom Welling) finding a goodbye video where she breaks up with him and explains that the world needs him more than she does.
In the same episode, Lex Luthor, who had just discovered Clark Kent’s secret, confronted Clark at the Fortress of Solitude and was presumably killed in their final showdown on Smallville.
The reason for this is tied to the fact that the actors’ contracts lasted until Smallville season 7. Tom Welling and Allison Mack extended theirs, but both Kreuk and Rosenbaum wanted to leave the show.
As a result, Smallville was put in a position where it had to rebuild its cast in season 8 by including Green Arrow (Justin Hartley) Lois (Erica Durance) and featuring Tess (Cassidy Freeman).