Why is the central fringe the brightest in diffraction patterns?

[MarcL]

The question is rather simple...

Why is the central fringe brighter in diffraction?

 

[Simon Bridge]

Brighter than what? The other fringes?

The brightness depends on the number of photons scattered from the location (or the electric field incident).
This depends on things like the available paths.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html
... basically the central maximum involves the simplest combination of paths - so they are more likely to be taken, so more photons per unit time get scattered.

The details depend on the level you want the answer at.
I suspect this will be hard to describe without math.