Question

why did single celled organisms gave rise to multicellular organisms

Answer 1

However, one mystery about multicellular organisms is why cells did not return back to single-celledlife. ... This kind of struggle is prevalent in the evolution of multicellularity because the first multicellular organisms were only a mutation away from being strictly unicellular.
Development of Multicellular Organisms. An animal or plant starts its life as a single cell—a fertilized egg. During development, this cell divides repeatedly to produce many different cells in a final pattern of spectacular complexity and precision.