Long story short. For breeder reactors getting neutron balance on to critical level is very challenging. You need one neutron to breed fertile material into fi

Long story short. For breeder reactors getting neutron balance on to critical level is very challenging. You need one neutron to breed fertile material into fissile and another neutron to split the fissile nucleus. Usually 2-2.5 neutrons are released in fissions of U-235, U-233 (bred from thorium-232) or U-239 (bred from U-239), but some of the neutrons are absorbed by other materials in reactor, some other neutrons are leaked out from the reactor core etc. I would say this is the main reason, why breeder reactors are not common. U-235 is the only fissile isotope available on earth so it was a natural choice for power generation.