The short answer is, because comic books and TV shows targeted towards children (Superman being sponsored by Kellogg's, to market their Frost Flakes and later plain Corn Flakes) there was public outcry about the content within them.

Robert Maxwell had already developed season one primarily off his radio program before he went looking for sponsors. When Kellogg's reviewed season one they insisted that the violence be toned down to make the show more kid friendly.

The most objectionable moments from 1951 had been removed in the re-editing of the series, and were not likely to be repeated no matter who sat in the producer's chair. Watchdog groups would ensure that macabre plots and sadistic villains would no longer be welcome. - ch 12.

When Ellsworth took over there was already outcry for season one. Writers such as Jackson Gillis tried to introduce more epic villains, but Ellsworth's editing veto prevented it.

Meldini is described int he teleplay as "a myopic European scientist" with an accent. Ellsworth's first change was to turn him into something out of a comic book. Gillis' scientist was far more grim and evil. - ch 12.

In April of 1953 there were Congressional Committee hearings on juvenile delinquency, with comic books and television programs being show cased and negative influences.

"Superman (with the big S on his uniform -- we should, I suppose, be thankful that it is not an S.S.) needs an endless stream of ever new submen, criminals and 'foreign-looking' people not only to justify his existence but even to make it possible." - Wertham's conclusion on comic book influences. - ch 13.

In September of that same year 24 publishers came together to create the Comics Code Authority, which used a template of DC's Code of Ethics originally created by Ellsworth and Liebowitz in 1940. It included the clause In every instance good shall triumph over evil and the criminal punished for his misdeeds.

Ellsworth may have thought he'd effectively toned down the violence after Maxwell, but now he even had to reign in his own taste. Banished forever were stories that might remotely be construed as horror or overly suspenseful, such as "Panic in the Sky", "A Ghost for Scotland Yard" and 'Lady in Black." Banished was any form of murder, both on- and off-screen, and if a character received a blow to the head, the scrip specified that it should happen "OUT OF FRAME,' with never so much as a lump afterward. Banished were criminal masterminds like the Wrecker, replaced for the most part with bumbling two-bit hoodlums. If one with a medium of intelligence happened to slip by, he was always paired with an imbecile. - emphasis added. ch 13.

That last quote is perhaps the most telling. Super criminals such as Lex Luthor were not allowed on the show.

Superman was now to be a children's show first, a family show second, with no risk of controversy. - ch 13.

Scifi villains such as Brainiac were somewhat out of character for the show. Other than the title Kryptonian, other aliens and out-world stories were at a bare minimum.

It wasn't until 1957 that would change.

By the summer of '57, comic book fantasy had replaced Davy Crockett heroism in the eyes of American youth. Shrinking men and hulking giants , prehistoric creatues and teenaged werewolves forced the singing cowboy from the B-picture scene. The logical solution was to let fast-paced action take a back seat to situations laced with science fiction overtones. For the first time, the producer would dispense with Mort Weisinger's services and simply co-write non of the scripts himself; - ch 14

That didn't change the general tone of the show and the insistence that Good must triumph over Evil. The show had also always featured Clark Kent and the Daily Planet with Superman coming in the end to save the day. To introduce regular super villains would have requirement more of the story to focus on Superman.

| All quotes are taken from Flights of Fantasy: The Unauthorized but True Story of Radio & TV's Adventures of Superman

Superman Super Site has this to say:

Neither Lex Luthor, Brainiac nor any of the regular comic book villains were used in the TV scripts, although a midget Martian, "Mr. Zero" (Billy Curtis) bore a vague similarity (in relative size only) to the comics' magical imp and recurring Superman villain Mr. Mxyzptlk. Carrying over the precedents established in previous electronic media productions of Superman, the bad guys on the TV show were usually generic thugs, evil scientists, Russian agents, crooked businessmen, or spies of fictitious foreign countries.

The claim that it was a precedent of established electronic media to not include the likes of Lex Luthor is patently false. As the 1950 Atom Man vs Superman has Lex Luthor masquerading as Atom Man against Superman.