What's going on?
Why do the 2 graphs look different? The Sun is a star and Virgo A is a radio galaxy. These two objects produce radio waves by different processes: thermal vs. nonthermal.
Thermal Radiation is essentially black body radiation. There is a peak in the intensity which is related to the temperature of the source and then the intensity decreases with increasing wavelength (See Figure 1). Stars are black body radiators. The Sun, though like most stars, is really close to us by comparison. That is why we can detect radio waves from the sun. Most other stars are too far away, and their apparent brightness at radio wavelengths is too low.
Figure 1. Characteristic curve for blackbody radiation.
Radio wavelengths aren't even shown on this graph! We would have to extend the x axis way off the page! But you can see, that for the right side of the curves, as you go to longer wavelengths, the energy decreases.
Nonthermal radiation can be caused by electrons interacting with magnetic fields. Electrons spiral around these fields, reaching speeds close to the speed of light. Since they are accelerating, radio waves are produced. This is referred to as synchrotron radiation. The intensity of synchrotron radiation increases with increasing wavelength. For example detecting synchrotron radio emission from Virgo A is easier at 20 cm wavelength than at 1 cm. Nonthermal sources of radio frequency radiation include: Jupiter, the Milky Way, pulsars, supernova remnants, radio galaxies and quasars.