Because the resolution power of electron microscope is much higher than that of the light microscope. Electron microscope uses a beam of electron which is a wavelength much longer than that of photons.

Importance of Electron Microscope in Cell Studies
The development of the electron microscope was a major breakthrough in the studies of cells because of the following reasons:
- Higher Resolution: Electron microscopes use a beam of electrons instead of light to visualize samples, allowing for much higher resolution images compared to light microscopes. This enables scientists to observe cellular structures in much greater detail.
- Visualizing Small Structures: The high resolution of electron microscopes allows researchers to visualize smaller structures within cells, such as organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum, which are not visible with light microscopes.
- Understanding Cell Functions: By being able to see cellular structures at a higher resolution, scientists can better understand the functions of different organelles and how they contribute to the overall function of the cell. This has led to significant advancements in cell biology research.
- Discovery of New Structures: The electron microscope has enabled the discovery of new cellular structures that were previously unknown or poorly understood. This has expanded our knowledge of cell biology and opened up new avenues for research.
- Advancements in Medicine: The detailed images produced by electron microscopes have contributed to advancements in medical research and the development of new treatments for various diseases. By studying cells at a more detailed level, scientists can better understand disease processes and develop targeted therapies.
Overall, the development of the electron microscope has revolutionized the field of cell biology by providing researchers with the ability to visualize cellular structures at a much higher resolution, leading to new discoveries and advancements in our understanding of cells and their functions.