What causes EM (waves) radiation to travel such a vast distances?A light ray from sun has a limited capacity?

It has to do with that simple law, an object will remain in constant motion unless acted on by another object or force. A photon emitted by the sun will travel forever, unabated and unchanged unless it hits something in its travel. A light ray from the sun, as you call it, has a limited capacity only because during its travel it will hit something out there. A few things will alter a photon. A chunk of matter (atom, planet, another star, etc.). An intense gravity field (immediate region around a black hole, a giant galaxy, etc.). An intense electric field (plumes of electrons shooting out of a neutron star). From the surface out to about 3 diameters, the sun acts like an infinite plane source. This means intensity remains the same. From 3-4 diameters, it acts like a line source. This means intensity drops off directly as the distance. Beyond 4, it acts like a point source. Point source means the intensity drops as the square of the distance. The numbers may be off (I'm relying on memory here), but the concept is true. I'll stop here. I think you get the picture.