Kristin Kreuk left Smallville after season 7, and only returned for a short arc in season 8 to wrap up Lana Lang’s story. For seven seasons, Lana served as Clark’s main love interest in the Superman prequel series. Once she was gone, Smallville was able to move on to a romance between Clark (Tom Welling) and Lois (Erica Durance).
Kreuk received second billing behind Tom Welling throughout the first six seasons, and was dropped below Michael Rosenbaum after that. Her presence saw a significant drop in season 7, with her appearing in a smaller number of episodes than usual. At the end of the season, Clark found a goodbye video from Lana, where she explained that she was leaving Smallville because she didn’t want to hold him back anymore. As a result, she wasn’t a part of the main cast in season 8. However, the show did bring her back for a five-episode story that ended with Lana’s body being infused with Kryptonite. Since he could no longer get close to her, Clark and Lana were forced to part ways forever.
Lana leaving Smallville marked a huge change for Clark, as she had held a major presence in his life. As for why the departure had to happen, the reason is related to the fact that her contract ended with season 7. Plus, Kreuk wanted to “try something else” and find other work outside of Smallville (via Metro). That made sense, given that she was starting to get movie roles during her final year on the show. When she was still a series regular, she was cast as the titular hero in Street Fighter: Legend of Chun-Li. Kreuk being busy shooting the movie was why she missing from four episodes in the second half of season 7. She also starred in Partition, a theatrical film which released in Canada in 2007.
Lana wasn’t the only main character to leave Smallville after season 7. Both Lionel (John Glover) and Lex Luthor (Michael Rosenbaum) departed the series as well, forcing Smallville to rebuild its cast. At this point, all that was left of the original season 1 cast was Tom Welling and Chloe Sullivan actress Allison Mack. Most of the characters who left came back for cameos and guest appearances, several of which being in the Smallville series finale. Lana Lang, on the other hand, never came back to the series. According to Kreuk, there was no need for her to return (via TVLine).
It’s true that the way Smallville moved Clark’s story forward made a return for her unnecessary, but it was disappointing to some that her exit had to happen, especially at that time. After years of Clark keeping his powers and Kryptonian heritage a secret from Lana, he finally told her the whole truth in the season 6 finale. Because of both this and her reduced season 7 role, Lana was only able to be a part of Clark’s small circle of allies for less than a full season. For many, it would have been interesting to see their new dynamic last longer, even though the knowledge that Clark would end up with Lois made a breakup feel inevitable from the beginning.