Probably because as exothermic reactions give off heat, they tend to be runaway reactions: once the reaction is triggered on one part of the substance the heat given off triggers it on more of the substance etc etc. Boom.
Because endothermic reactions soak up enery they have the opposite effect - stifling the reaction, shutting it down. So there needs to be a continual supply of free energy to keep the reaction going. This is much rarer in nature.
This is a guess btw, but it makes sense to me.
Answered Thu 11th October, 2012 @ 01:31 by Robotperson Edited by Robotperson on Thu 11th October, 2012 @ 01:32