Can a abscessed tooth show up on a cbct scan, but not show on a 2 dimensional dental xray? if so why?

2D images show less: Since there a many more different views of the areas around the root of a tooth with CBCT, it is much more sensitive than a single plain film "dental" xray--a single view of the area. However, NEITHER is necessary to diagnose an abscess! To properly diagnose an abscessed tooth depends on physical findings (pain, percussion and temperature sensitivity, and swelling), rather than "X-rays".

Bone density: Simple answer: Bone density can mask an abscess on a 2D image. Think of a 2D image like a photograph of an object and a CBCT as holding the object in your hand. One you can spin around and see from every angle, the other you only get to see one and in one frame. Night and day difference.