As far as I can tell, there is a simple and elegant explanation for the discrepancy: Rudolph (and Breivik, and most of the present crop of right-wing terrorists) is an intellectual zero while Kaczynski is not.

If Jack the Ripper or Jeffrey Dahmer had left behind something like Kaczynski's manifesto (and later prison essays), some respectable intellectuals would pinch their noses and study them.