Why was the traditional two-kingdom classification considered inadequate?
The two-kingdom system did not distinguish prokaryotes from eukaryotes, unicellular from multicellular organisms, or photosynthetic organisms from non-photosynthetic fungi. Many organisms therefore did not fit properly into either Plantae or Animalia.
Ref: NCERT Class 11 Biology Chapter 2: Biological Classification Five Kingdom Classification and Kingdom Plantae Animalia