That is a quite difficult question to answer it in few lines. The main reason will be the difference in the scattering factor of different atoms, as well as the so called "structure factor". I would suggest the book "Crystals and crystal structure / Richard J. D. Tilley." for a simple but complete introduction.

The answer to this is the reason we can determine structures of materials using X-ray crystallography. The direction of diffraction is determined by the inter-planer spacing in the crystal lattice, but the intensity is determined by the scattering power of the atoms and their relative positions. This leads to both constructive and destructive interference, thus different intensities.

Thank you very much to all of you for this reply