Answer:
Because there is scientific evidence that evolution occurs gradually and endocimbiosis is a type of evolution that occurs suddenly.
Step-by-step explanation:
Although Dr. Margulis was very thorough in her defense of endosymbiosis, she had to face great resistance from the scientific community around this theory. This was because as we know, and as it is scientifically proven, evolution is something that happens gradually and slowly and that is exactly what scientists believed and defended. However, endosymbiosis presented a kind of evolution that happened abruptly and suddenly, which provoked resistance to the theory.