Superman the movie is based on the silverage comics in which Lana doesn't know the truth.

Smallville is based on post crisis which is where Lana does find out the truth. The writers wouldn't toy with Lana being so close to finding out and have that be her big storyline if she doesn't end up finding out sometime on the show.

Episodes like Truth factors on why Chloe will never find out the truth about Clark... she just can't be trusted. While Chloe off saving her thoughts on Clark on disk, computers and out loud to Lionel.

Lana keeps everything she spec between her and Clark.

That's what sets Lana away from Lex, Chloe and Lionel. When it comes to the truth about Clark.