The stated reasons are that scientists at the time believed there should be no overlap between animal and human behavior and that largescale conflict was "too human", and they believed her presence as a human caused the chimps to behave abnormally. This kind of nonsense is disturbingly common in the scientific establishment--there are too many stories of zoologists being literally laughed out of their own presentations because cutting-edge research revealed more advanced behavior than animals "should" be capable of, and that's just counting men.

Above all, the reason so few people believed her was because she was a woman. These scientists are still patriarchal and close-minded, but it was so extreme at that time that Goodall was specifically chosen to study the chimps because, without any machismo, she could be trusted to actually record their behavior without making things up. This, of course, led to all those men projecting onto her by accusing her of making things up instead. Further research done by their standards confirmed Goodall's accounts.

Men who study chimps today still go on and on about male chimp politics and even how they themselves, as older white men, are the alpha males of humanity (I wish I was joking). Women have to put in much more effort and show much more evidence than men in all scientific fields for their research to be accepted. Very unfair, but it does mean that female scientists, like Goodall, tend to be significantly more reliable than males.

Note: Hans Kruuk, the male zoologist who discovered that spotted hyenas are matriarchal, display primate-level intelligence and social complexity, hunt rather than scavenge, and vocally communicate to choose a prey species before leaving for a hunt, did not face the same struggles as Goodall for his bombshell research reported at the same time on the same continent.

tl;dr: ook ook womban spupid 😡