Power and cost.
Though the majority of Valkyries were out intercepting the Ragnarok Heralds and the hoards of Calamities they led, it was impossible to stop every last one of them from advancing and despite the girl's efforts some ended up slipping through the cracks.
The numbers weren't in humanity's favor. There was no end to the mindless drones of the Nephilim unleashed on their enemies, and though they only used the most basic forms of arts, that was enough to give anyone but their owners a very hard time.
[FIRE!!!]
Following Nana's signal, seven of the Valkyries guarding the forty sixth base fired their cannons at the approaching swarm of calamities. A series of explosions shined in the distance, but even though the attack took down quite a few drones the rest flew right through the rain of fire as if it didn't affect them in the least.