Are computers illegal in Dune?

The lack of “thinking machines” in Dune is intentional, and the ban on this technology actually creates the basis for all of the world-building within the original six Frank Herbert novels.

Why was Ai destroyed in Dune?

It was realized inadvertently with the creation of the thinking machines, whose destruction eventually came during the traumatic Butlerian Jihad. It was abandoned as a concept after the costly Jihad, and remained a pursuit that was actively condemned by the Landsraad via the Great Convention.