This is just one of the things that never seemed to make that much sense to me and I think that has more to do with making the Minbari overpowered period. I mean, for God's sake, Delenn's White Star group took out a whole Drakh raiding party in Lines of Communication. While the White Stars are indeed impressive, the Drakh LIVED on Z'ha'dum with direct access to ALL of the Shadows' technology. There is no way that they should've been destroyed as easily as they had been. At best, it should've ended in stalemate.
Don't get me wrong, I like the Minbari, but for the love of God, there should be races outside of the First Ones that can give them a run for thier money. From what was mentioned in the Legions of Fire Trilogy, it may have well indeed been the Drakh, since the plague was originally going to be used on Minbar after the Shadow planet killer destroyed Earth. Other races would include the Third Space aliens (but seriously, these guys fall into the First Ones category seeing how they one-upped the Vorlons for a while), and The Hand (well that's more speculative at this point since the only thing we saw were a few small attack craft blow apart the Valen and damage the Liandra).