I feel like one big reason they didn't make this discovery in the past is how would they stab a herald with it. Even if you have lets say ten raysium weapons you have to hit the herald while they obliterate your ranks and are backed up by an army. Stabbing herald with a killing blow with a weapon there are very few of and not knowing what would happen would be much harder to convince other fused whereas trying to then stab a drunk insane herald with no one helping them is much easier to convince other fused. Also no one expected such powerful entities to be able to be held in a gem till Ba-ado-Mishram