Although the Merit Stele allowed them to summon who they could afford to, it was both a lengthy and uncertain process.
Lengthy because the summoning wasn't automatic, its speed depending on the number of sacrifices provided. Uncertain because the summoning was by itself the reward. Nothing guaranteed that whoever was summoned would fulfill the summoners' wishes. On the contrary, if summoned without warning, Higher Realm Devas and Demons were very likely to obliterate the summoners.
For that reason, using a substantial amount of merit, Anatol first spent several years to establish a dialogue with his chosen houses, and negotiate their support. Only after receiving their agreement did he start the ritual.
But following the first confrontation, even he was starting to doubt their ability to endure until the reinforcements' arrivals.
I just realized that although it started yesterday, due to the current system, you folks probably don't see Volume 3's title yet.
Volume 3: Resurgence of the Zenith Ants