Most of the Smallville actors have been doing steady work since they left the show but its usually low profile shows in Canada and small indie films or cable shows that no one sees (Rosenbaum is on Impastor on TV Land, John Schnider on Haves and Have nots on Harpo, Annette O' Toole on Halt, Catch and Fire, Allison Mack on Wilfred, American Oddessy, the Following). Welling has been doing quiet small appearances in movies (Parkland, Draft Day, The Choice. He was supposed to have his own show on CBS this year that he produced playing a cop, kinda same deal Kristin had with the final season of BATB and Erica with Saving Hope where they give them the producer reigns). Laura Vandevoort and Justin Hartley and the Ashmore kid have probably done the most steady low pro TV work
I dont think its an acting issue, there's worse actors getting work steady, I honestly think working on Smallville for 7-10 seasons burned a lot of them out and lost interest in doing stuff. Welling and Kreuk kinda hedged on their work and turned down stuff. Rosenbaum did stuff that was not major just to work with friends (Breaking In, Back in the Day)
That show really had terrible writing and when you look at these new superhero shows even at their best they make the same mistakes of repetitive arcs and I think that turns them off grinding and doing everything. Most have got insanely paid in royalties for the show so thats why they are all looking at producer based stuff funding their own projects (Which never gets released)