Same reason high visibility jackets are I presume. I always think they're green but others tell me they're yellow.
I have normal color vision as do at least some of the people who see them differently
I always just assumed school buses were yellow because they're always referred to as yellow, but then my 3 year old child said "look at the orange bus" and as I looked, I realized that indeed the colour of a bus appears to be more orange than yellow. Now I can't un-see it.