I take it that you are using the material preview mode? This is occurring because the material preview mode still applies lighting to your model albeit not much. When the light strikes the darker and lighter areas of the texture, it uniformly brightens them all since there is no geometry/normal map etc. to cause the shadows to be emphasized or maintained.
Try increasing the material roughness and reducing the material specular properties if you want to minimize the effects of lighting.
Just for fun, take a sphere and set roughness to 0 and metallic to 1 in the Material Preview mode. This is what you'll see since Blender is using some lighting to light the model.