Its an interesting question you present. Not only 'Why that paininting?"? but also "Why pick that person?"
Why not the guy in his toga sitting on the stairs? thats just as interesting.
The question why Kostabi picked those precise figures is indeed interesting. Most of the people featured in School of Athens have been recognized as various philosophers, yet these two feature in the Kostabi painting remain, infact, unknowns. The person you're referring to on the stairs is Diogenes, "
lying carefree on the steps before them to show his philosophical attitude: he despised all material wealth and the lifestyle associated with it". If you look at the Kostabi pair in the original painting, what do you see? Most people next to them are attending to the debate of Plato and Aristotle. To keep it simple, Plato is pointing upwards, while Aristotle is gesturing downward; this is to portray their philosophical differences. Plato would believe in the Forms, while Aristotle put more weigth on empirical knowledge.
The Kostabi pair has one person making notes, similar to several other people in the painting. His hair appears to be swirling, yet there's no wind to be seen. One could see him caught mid-motion, setting his gaze down to the notebook in a hurry. So it's quite possible he's observing the ongoing debate. The person next to him appears not to give the event much thought, and seems rather disassociated of his environment. He's leaning on the railing, appearing to have a some sort of a leisurely position. Hence, if you consider these two figures as a pair, you might end up with the idea that the younger one is busy scribbling some thoughts down for future perusal, while the older, more seasoned one is considering him somewhat cynically; as in "use your illusions, but I doubt they'll do you much good in the long run."
That's really just my interpretation of the Kostabi painting, which I believe should be looked at in joint with School of Athens, as it is a partial reproduction.
Here's a Wikipedia entry for School of Athens;
here's a character breakdown.