Smallville was the most successful CW DC show, and the longest-running superhero TV series, before the Arrowverse came along. This Superman prequel series was a last refuge for the Man of Steel during a time when he didn't have any live-action movies or animated shows. Smallville's success led to its introduction not only its own versions of characters from the Superman mythos, but several other unrelated DC heroes as well.
These included interpretations of Cyborg, The Flash, Green Arrow and Aquaman, who briefly formed a sort of prototypical Justice League. One character that Smallville failed to properly introduce during its TV run, however, was Batman. Due to Warner Bros.' ambivalence to allow disparate versions of the Dark Knight to appear in live-action, Batman never came face to face with Tom Welling's young Clark Kent.
Why Batman Never Appeared in Smallville
Batman was one of several characters who was planned to show up in the series at some point, even if it was just as a young Bruce Wayne. Showrunners Al Gough and Miles Millar were consistent in their desire to introduce the character in some form. His absence became even more egregious given all of the other DC characters brought into the show, namely Bart Allen and Victor Stone, who Superman had no real relationship with in the comics.
The reality behind the lack of Batman was finally revealed in 2007 at a Los Angeles Comic Book and Science Fiction Convention. There, Gough and Millar made it clear to fans that they had tried several times to get Batman included in some official way in the series. Holding them back, however, was Warner Bros. themselves. Batman had finally become a hot commodity again thanks to the success of Batman Begins, and things only continued to heat up with the release of The Dark Knight. Warner Bros. wanted to maintain the character for the film series, and didn't want to confuse viewers by making them think that Smallville was connected.
Similarly, as revealed at the convention, the still recent Superman Returns and its portrayal of Superman and Lois' relationship kept Gough and Millar from doing certain things with the characters in Smallville for a few seasons. Because of this, it was only towards the show's final seasons that the two were cemented as a couple.
The Bat Embargo
This all stemmed from a concept referred to by fans as the "Bat Embargo," which dictated that certain Batman characters could not be used in certain productions. This was due to the slowly revitalizing film franchise for the character, as well as other cartoons being developed starring him. A similar issue occurred when the Arrowverse killed off its version of the Suicide Squad because the film was set to release.
Despite his absence in the TV series, Batman's presence was still felt in Smallville. Lex Luthor had a vision early on in the series showing a corpse wearing the iconic Batsuit. Likewise, Adam Knight, an original character who was prominent in the third season, was initially believed by fans to be a mere pseudonym for Bruce Wayne. This stemmed from his martial arts and computer skills, his being an orphan and Adam Knight potentially being a combination of Adam West and the Dark Knight.
Green Arrow, much like his later Arrowverse incarnation, was also used as a Batman expy. Green Arrow and Superman are not close at all in the comics, but the two are essentially the Smallville continuity's World's Finest. Chloe Sullivan would also make allusions to having met a "billionaire with hi-tech toys" in her travels, obviously referring to Batman. The Dark Knight proper finally showed up in the Smallville: Season 11, the comic book continuation of the show.