The Pillars were given their name because that was exactly what they were to the Behemoth Clan. They were the members that allowed the whole clan to be declared one of the three major clans in the Demonic Faction for a reason.
The sheer size as the biggest clan, maybe having more members than the other two major clans combined, was just an added threat to the clan's power. The real threat of the clan and the reason why it was able to be considered a major clan was because of the Pillars and Sha Mo himself.
Two had been taken out in the last attack against Flendon. Knowing failure wasn't an option this time, not only did Sha Mo not send two more of his Pillars, but he had also come himself. The main targets: the Crimson Crane and the Dark Magus.