Way I see it, itís because this is the internet. New people arrive to D&D, to the world wide web, and then to the intersection of such every day, and they may not know about the edition war. Thus the entire thing kicks up again when someone decries the newís guyís edition of choice, without realising that battle has already been fought a hundred times.
TLDR: Why not just stop fighting the edition war?
Not quite a vicious cycle, but nerveless this long-running flame war keeps going because thereís always fuel for the fire.
(Also personally, I prefer pathfinder to 4th ed but I donít mind 4th either. For one, the mechanical-fluff interplay of the monk works perfectly, and they are now the mobile fist-fighting wuxian they were always meant to be. Also I like the ideas behind some of their classes, like the seeker. Itís just that I wish you could decrease stats in point-buy so that you could take advantage of a race with bonuses in stuff you donít need and thus play against type. Itís very bad at that.)