Why do glial cells of the retina become more abundant the closer they are to the optic nerve?
Any particular glial cell you're noticing this with?

I know that Astrocytes modify Calcium concentrations in the synaptic cleft. Perhaps at places where there are more synapses, more Astrocytes must exist to regulate such thing? Here's an interesting paper on a possible role of Astrocytes on associative networks:

http://www.antanitus.com/