Question

Why does the intensity decrease in single slit diffraction?

Answer 1

At the center all the huygen sources from the slit opening interfere constructively and give you the maxima. 

But as you move away from the center, and move towards the first order maxima, some huygen sources kill each other, and the remaining construct to give light. So it wouldn't be as bright

Also if you go further out, the intensity becomes even smaller..