The easiest way to describe what cases like what powders is this:
Expansion ratio dictates how a powder burns in any cartridge/calibre.
Powder burning rate is not constant from case to case. The 338 performs best with RE19 because it burns correctly for the case size and bore size, which is dictated by it's "Expansion Ratio". This is the measurement of bore volume in CI divided by case volume in CI.
The reason is this, because there is more area under the bullet this powder burns progressively below it, whereas a slower powder has trouble burning progressively and may not even reach max pressure before the bullet exits the muzzle. It has been deemed too slow for the application.
Essentially, the combustion chamber is larger and therefore allows medium powders to behave like those of a slower nature.