Infecting Zaen took the highest priority for the broodlings. They desperately wanted access to the 'system' he possessed. Once they did, they'd get rid of their only weakness, which was literally, weakness.
Unlike other possessing anomalies, Broods had only the ability to occupy a being. Moreover, they were like cancer. The longer they stayed inside a host body, the weaker it got.
Now that the broodlings had infected others for months, naturally they were quite weak. However, with Zaen's system, they could effectively 'replenish' their stats.
They never thought that their only strength of possession would prove useless against him. The system was effectively rejecting them. Whenever the worms invaded his body, they'd die and instead heal him.
Now that they know they couldn't infect him, the only thing remaining was to dissect him and see what made him different from the others.
