Q.Why electron is preferred over X-ray in microscope ?

Electrons are preferred because their de Broglie wavelength (and hence resolving power) is much better than that of X-rays of comparable energy, and because electron beams — unlike X-rays — can be easily focused using electric/magnetic lenses.

1. Resolving power / wavelength: the resolving power of a microscope improves as the wavelength used decreases. Electrons accelerated through even a modest potential (tens of kV) acquire a de Broglie wavelength ($\lambda=h/\sqrt{2meV}$) far smaller than the wavelength of X-rays of comparable energy, so an electron microscope can resolve much finer detail than an X-ray-based instrument.
2. Focusing: a beam of electrons, being charged particles, can be easily converged/focused using suitably shaped electric or magnetic fields (electromagnetic 'lenses'), exactly as an optical lens focuses light. X-rays, being uncharged electromagnetic radiation of very short wavelength, are extremely difficult to focus with simple lenses (they barely refract), so forming a magnified image with X-rays is impractical. …