Passage: Six months later, Drake's symbiosis trials are closer to success, though one of his symbiotes dies due to carelessness. Brock is approached by Dora Skirth, one of Drake's scientists who disagrees with his methods and wants to expose him. She helps Brock break into the research facility to search for evidence, and he learns that an acquaintance of his, a homeless woman named Maria, is one of the test subjects. Brock attempts to rescue Maria, but the symbiote possessing her transfers to his body without him realizing, leaving her dead. Brock escapes and soon begins displaying strange symptoms. He reaches out to Weying for help, and her new boyfriend, Dr. Dan Lewis, discovers the symbiote on examining Brock. Drake exposes Skirth to the remaining captive symbiote, which ultimately dies. This leaves the symbiote inside Brock as the only known surviving specimen.

Question: Why did one of Drake's symbiotes not survive?
carelessness