It is a matter of simplicity of optics, damage to the sample and contrast. Visible light optics are very advanced (i.e. glass lenses), but it starts to get difficult as you head towards shorter wavelenths. X rays, especially high energy (short wavelength) ones, are extremely hard to focus. Short wavelengths of light also damage biological samples (imagine UV and sunburn). A key requirement for generating an image is high contrast, use of very short wavelength light/electrons requires heavy metal staining to get good contrast, not exactly ideal for looking at a living cell.