The wheelchair was lead-lined.
The wheelchair was lead-lined to hide the bomb. I'm not sure that this was clarified in the theatrical cut but I'm pretty sure it was explicit in the DVD/Blu-Ray releases. No need to lead line a wheelchair unless you're hiding it from x-ray vision.
...Later in the film, she [Jena Malone, playing a forensic scientist] calls Lois to say she did an analysis of the materials in Wally’s explosive wheelchair and found that it contained …… lead! After Superman appears before a congressional subcommittee to talk about the Nairomi incident and Wally’s IED-chair kills every human inside, Supes wonders why he didn’t see the bomb. Now we know it’s because the vehicle was lined with lead, which the Man of Steel traditionally can’t see through.
Vulture.com re Ultimate Edition Release
How the explosive evaded human detectors etc (plus wouldn't a lead-lined wheelchair be somewhat suspicious when x-rayed?) is not clear but it's possible the explosive was a new LuthorCorp invention not detectable using the technology available to Capitol security.