The image shows the intensity vs 2-theta plot of X-Ray diffraction pattern obtained from an aluminium target. Why is it so that as the 2-theta increases the peak intensity decreases?
The intensity depends on a number of factors, mainly the structure factor, multiplicity of plane, Lorentz factor, polarization factor, temperature and absorption factors. Combination of all these factors results in intensity variation.
To simplify things a bit, the most important factors relevant to your question are the Lorentz factor and polarization factors. If you follow the links, you'll find simple angular relations for these factors, which should answer your question.