The character (https://tvtropes.org/pmwiki/pmwiki.php/Characters/SupermanLexLuthor) was first introduced in April 1940, which was at least, a good 12 years or so before The Adventures of Superman debuted. So you can't say that it's because the character wasn't created yet.

Either way, not having Lex Luthor on a Superman TV series (https://www.answers.com/general-arts-and-entertainment/Why_did_Lex_Luthor_never_appear_on_the_1950s_Superman_TV_show_with_the_late_George_Reeves) would be like making a Batman TV series and not having The Joker in it as a recurring villain. And funny enough, the latter Batman TV series with Adam West would actually showcase The Archer (https://batman.fandom.com/wiki/The_Archer) (played by Art Carney), who originated as a villain in the Superman comics (https://tvtropes.org/pmwiki/pmwiki.php/Characters/SupermanRoguesGalleryAToL) in December 1941 and The Puzzler (https://batman.fandom.com/wiki/The_Puzzler_(Dozierverse)) (played by Maurice Evans), who first appeared (https://en.wikipedia.org/wiki/List_of_Superman_enemies#Foes_of_lesser_renown) in the Superman comics in June 1942.

Of course he should have appeared, but readers of modern superhero comics and followers of films need to realize the following fact: in the 1940s and 1950s, DC's superhero comics were not anywhere near as super-villain oriented as they later became. They are now super-villain DRIVEN. Readers going back to the old comics often complain about the lack of heavy-hitter villains back then.

Luthor and The Joker appeared at times, but as a rule super-villains were not at all emphasized in the way we are now used to. There were long stretches of time when they didn't appear in Action/Superman and Detective/Batman. (In the LATE 1950s, DC did a memorable Luthor-Joker team-up in World's Finest.)

Look at The Riddler for example, he had no comics appearances between 1948 and 1965. The Scarecrow similarly.