How can countries let genocides occur?

It is not enough to say that they occur because of fear or hatred among groups. Fear and inter-group animosities exist in many places, but genocides occur only occasionally. Fein calls this a "retributive" motivation for genocide. When one group is convinced that its existence is in jeopardy from another group, it may feel that they have no other choice but to destroy the competing group. This boils down to a "we have to get them before they get us" attitude.