An extremely low interaction probability
The neutrino probability of interaction (cross section) with a proton or neutron increases with energy. The data shown of the figure represent this interaction probability increase with energy. The interaction probability is extremely low for the neutrinos produced by radioactivity. Owing to their high energy, it is much easier to detect neutrinos foundwithin the beams of large particle accelerators.
Image credit: J.W. Rohlf, retrieved from http://hyperphysics.phy-astr.gsu.edu