Why did Dr Margulis met so much resistance with her introduction of endosymbiosis?

Dr. Lynn Margulis faced significant resistance to her endosymbiotic theory primarily because it challenged the prevailing views of evolution and cellular Biology at the time, which emphasized competition and individual organism survival. Many scientists were skeptical of the idea that complex cells (eukaryotes) could arise from symbiotic relationships between simpler cells (prokaryotes), viewing it as an unconventional and radical departure from traditional evolutionary thought. Additionally, her ideas were initially met with a lack of empirical evidence and acceptance within the scientific community, leading to prolonged debates before they gained wider recognition and validation.