My guess: Because you can dye a white belt black, but not vice versa.
I had a friend that did judo, and every time he advanced he dyed his belt to the new colour. The order being convenient for that because the colours get progressively darker (in Canada the order is white, yellow, orange, green, blue, brown, black).
Traditionally, it was said that after all the work you put into going from a white to black belt, your belt would be so dirty that it would just become black. Now, things have changed. We have degrees or dans, and they must be put on by hand/machine. Some dojos give you a black belt when you’ve reached that level in your training and others unfortunately just want your money. Either way, you’re not supposed to wash it because of the “ki” and it stands for all you’ve worked for.