Well then I think that page has the answer. If most of the electrons basically die out and give a weak signal because they hit the outer electrons and slow down, then you are only left with those really strong repulsed electrons that "bounce off" the crystal structure at certain angles.
Mind you, I know absolutely nothing about X-ray crystalography accept the pictures I have seen in the textbooks, but I can draw a picture in my mind that may explain it.
Say you have a door that is tilted at an angle in front of you, 45 degrees. If you have a bucket of baseballs and start throwing them at the door, they will mostly all bounce off and go in the same direction. Yes, many will slow down, some will go slightly higher or lower than others, but they will all have the same trajectory leaving that door, basically like the angle of incidence = angle of reflection. If you have trillions of electrons bouncing off in the same direction with about the same amount of energy, I'd think you get those really bright bands we see so often in X-ray crystal pics.
I don't think you get the crystal structure from where electron waves had constructive or destructive interference like in the double slit experiment, I think you get them because that is where the most electrons are accumulating and bouncing too.
I view electrons as particles in this particular case, although an in depth discussion about how x-ray crystalography reveals the wave-particle nature of matter would be of interest if there is anything out there about that. My school has poor inorganic labs, as I think most others do....ive never heard of an inorganic lab class