Maybe they are afraid of opening a Pandora box of answers that would crash everything that we consider to be true about our origin - meaning life on other planets, or even provoking a much more advanced civilization with our presence.
Actually, with my limited imagination I can't think of anyone being afraid other than religionists, whose "Garden of Eden" would become much more of a fairy tale than it is now. Well, universe certainly is a big place, and much more mysterious than out telescopes or microscopes can tell, so we don't know what is there bigger than the biggest and smaller than the smallest. Considering how much is still there to discover, science is still in diapers.
I, for one would just love some gigantic discoveries to be made in my time.