Q:

Why didn't Netero call the more powerful adventurers for help in Chimera Ant arc?

In the Chimera Ant arc of Hunter x Hunter,

 Netero and the Chimera King Meruem have a showdown, which leads to Netero's death, and later on, Meruem. The fight was pretty one-sided, considering the final state of Meruem compared to Netero, but it was to expect considering the former events.

We learn later that there are powerful adventurers out there (the Zodiacs), that have a seemingly tremendous amount of power.
My question is: why didn't Netero call them for help? He knew the enemy had a tremendous power, and he may have avoided what happened to him if he was helped during the mission (not even the showdown). So why did he call only those who are part of the mission in the Chimera Ant Arc? This could have been way less dangerous for them to have more firepower and capabilities.

A:

I don't believe this question was directly answered in the anime.  (I don't know if the manga provided an answer or not.)  So I'll describe my understanding from context.  The extent of the problem was not understood immediately.  I believe Kite was the best hunter to face the ants originally.  The other hunters tended to be new, such as Gon, Killua, and Pokkle. That ... didn't go well (for the hunters).

 Kite and Pokkle were killed. Nen information was gathered from Pokkle's brain, and Kite's reanimated body was used for training of the ants.

After that, new hunters were kept away, and more experienced hunters were sent to gather information and make a plan. They seemed to have been led by Morel and his crew, Knov, Knuckle, and Shoot. Gon and Killua were not allowed to reenter until they could prove themselves to Knuckle and Shoot.  I think they were given a month. In the end, those four and Palm were allowed entry.
A plan was accepted and modified by the Hunter Organization, and we saw what part the aforementioned folks did within the ant-claimed territory. We didn't see the whole plan, though. Not even Morel knew the full extent of the plan.

 Netero prepared himself with a poisoned nuclear device ("rose") implanted within his body, and he hired Zeno Zoldyk to make a grand entrance with him at the ant King's castle. Netero probably knew he would have to use the rose, although I'm sure he hoped he could beat the King without it. Netero intentionally gave up his honor in his fight. The only honorable thing he did was tell Mereum his name. Mereum made multiple attempts to discuss co-existence with Netero, and in their fight he only acted defensively. Netero ignored that for fear of losing his resolve to win at all costs.

I don't believe members of the Zodiac would have been helpful. They were indeed powerful, but their talents did not fit the plan. Ging may well have been rejected from consideration outright, being thought a loose cannon. He could not be depended on to follow the plan as events on the ground evolved, especially if a peaceful resolution should present itself.

