In the following article, under the section Physics of the Superlens, it is stated:
The light emitted or scattered from an object includes not only propagating waves but also evanescent waves, which carry the subwavelength detail of the object. The evanescent waves decay exponentially in any medium with a positive refractive index so that they cannot be collected at the image plane by a conventional lens, and this results in a diffraction-limited image.
Why do the evanescent waves need to be collected? How do they cause the diffraction limit? Non-technical as well as technical answers are appreciated.