FATE rewards are decided by the amount of enmity generated by you and your party during the FATE, it is also proportional to the total amount of enmity generated by everyone present in the FATE. It is also graded a lot more strict in special HNM-style FATE.
The more people are there (and in Behemoth, there are hundreds) and the rarer the FATE, the harder you have to work to get graded.

It is not the games fault you do not understand the mechanics.