Smallville won't be included in The CW's Crisis On Infinite Earths crossover, and there are a number of reasons why it's proven so difficult to make Tom Welling's Superman universe part of Arrowverse canon. The upcoming crossover will be a truly monumental event that promises to tear apart the Multiverse, but it will also feature appearances by past DC superhero icons like Burt Ward. Perhaps most exciting is the prospect of Brandon Routh portraying the Kingdom Come Superman and appearing alongside Supergirl's Man of Steel, played by Tyler Hoechlin.
Since crossovers are inherent in the Crisis, it's hard to blame fans who are clamoring for the appearance of Smallville's Superman as well. For a generation of fans, Tom Welling's Clark Kent is the definitive Man of Steel, even if he never technically wore the iconic Superman costume. The flagship series of The WB/The CW network for a decade, Smallville has been a major influence on the Arrowverse and on superhero TV series in general. It pioneered recasting DC's comic book icons to be younger and sexier, and it unabashedly provided fan service by guest-starring the late Christopher Reeve and Margot Kidder as well as showcasing its version of the Justice League, Justice Society, and the Legion of Super-Heroes.
However, even if Smallville was granted its own Earth in the Multiverse it's still perhaps the most awkward series to meld into Arrowverse canon. The sheer number of conflicting versions of characters and actors from Smallville who have since been cast in the Arrowverse could become confusing for viewers: this includes Erica Durance, who was Lois Lane on Smallville and is now Supergirl's mother Alura; Smallville's Supergirl Laura Vandervoort played the villainous Indigo in Supergirl; and Sam Witwer, who was Smallville's Doomsday, became the Supergirl villain Agent Liberty. While Durance's Lois would likely be the only core Smallville character fans would want to see appear in the Crisis, it would still require some sort of explanation why she looks exactly like the Kryptonian mother of Kara Zor-El (Melissa Benoist) in Earth-38.
Smallville's actors, who are all nearly 20 years older than when the series began, have also moved on. Tom Welling, who just starred in Lucifer, is perhaps the most reluctant to portray Clark Kent again. Indeed, Welling has said he'd be much more interested to play Batman than go back to playing the farm boy from Kansas. Michael Rosenbaum was the definitive Lex Luthor for many fans, but he also recently debunked appearing in the Crisis. Kristen Kreuk, who played Lana Lang, is more open to an animated Smallville revival. Meanwhile, Justin Hartley, Smallville's Green Arrow, now stars in This Is Us.
Unfortunately, Smallville in the Arrowverse would also have to deal with the absence of Allison Mack, who is in massive legal trouble and plead guilty to racketeering and conspiracy charges for her role in the NXIVM sex trafficking ring. It's already awkward because Mack's Chloe Sullivan was a primary Smallville character; indeed, even Supergirl's version of Superman grew up in Smallville with his own Chloe Sullivan as his best friend who had a "Wall of Weird". It would be a challenge, to say the least, to explain Chloe's whereabouts in any Smallville revival.
Since Arrow debuted a year after Smallville ended, the Arrowverse has always tried to differentiate itself from its predecessor. This includes doing the opposite of Smallville's infamous 'no tights, no flights' policy and fully embracing DC's superheroes with comics-accurate costumes, superpowers, and tons of fan service. No doubt if Smallville were to become part of Crisis On Infinite Earths, fans would look at it as the long-awaited sequel to Smallville but even an event as big as this crossover would have to focus on the Arrowverse's many main characters, leaving little time to properly explore what the last decade of Superman's life post-Smallville has been like.
While it would be a dream come true for Smallville to be part of the Arrowverse, Crisis On Infinite Earths may not be the right place to integrate that series into the shared universe; it would be too difficult to properly give Smallville its due and would likely leave those fans disappointed and wanting more. Not to mention Smallville's Earth possibly being destroyed in the Crisis isn't something fans want to see.