Remember it was originally a Euro standard. White and black were already in use in some countries. So it may have been a question of using as few colours already in use as possible. If only to avoid the 'Germany has got its own way again' or whatever that you'd inevitably get on here.
If you reduce light blue and brown to monochrome, the difference is obvious. Dark blue might have looked the same as brown.
My view is those who chose the eventual colours did so with considerable skill after looking at all the alternatives. And that it's impossible to satisfy everyone.