Here's why:

* Differentiation is a process where cells become specialized, taking on specific structures and functions. This is a crucial part of development.

* Development encompasses the entire process of growth and change from a single fertilized egg to a complex organism. It includes differentiation, growth, morphogenesis (shape formation), and cell death.

While differentiation is a key component of development, using the term "development" is more accurate when describing the overall process of an embryo becoming an organism.