Question

why two kingdom classification was used for a long time

Answer 1

the reason was our science was not that much devolped. and second was there is huge no. of species in earth

Answer 2

before we faced difficulty in distinguishing lower level of animals and plants or fungi or plants or bacteria and plants there was no difficulty. but once this problem qas faced a system of classification was required which was the 5 kingdom classification by R.H.Whittaker. but it was long after the discovery of 2 kingdom classification as these confusing organisms were discovered much later.

hope it helps u