If the Rose Bomb is so powerful and poisonous, why didn't the hunter association airdrop a dozen on the palace when fighting against the ants?
Netero wanted to settle the matter as a Hunter first, but implanted the bomb within himself as a contingency plan. He went in personally and fought Meruem. He lured Meruem away and fought him solo in the desert, when it was clear that he could not win, he killed himself, triggering the bomb a safe distance away from any casualties.

Also, HxH is a shounen series and they had to settle it the shounen way, with a 1v1 fight.