Radio dishes have a metal mesh surface that lets some of the radio waves through the back of the telescope.
Using many radio dishes provides the ability to image the object under question from many points of view simultaneously.
Radio dishes cannot be built with perfectly smooth surfaces.
Radio dishes are difficult to build larger than a few tens of meters across, limiting their individual resolutions to several times worse than the largest optical mirrors.
The collecting area of several radio telescopes added together is much larger than that of a single radio telescope.