Final answer:
Gram-negative bacteria do not retain crystal violet during Gram staining due to the presence of an outer membrane, which is not found in Gram-positive bacteria with thick peptidoglycan layers.
Step-by-step explanation:
The reason Gram-negative bacteria do not retain the crystal violet of the Gram-stain is because they have an outer membrane that surrounds the cell wall which is thin and composed of peptidoglycan. In contrast, Gram-positive bacteria have a thick peptidoglycan layer and lack this outer membrane. Therefore, the correct answer to why Gram-negative bacteria do not retain crystal violet during the Gram staining process is because they have an outer membrane.